getServerSideProps should support props value as Promise

[kara]

Previous to this change, getServerSideProps could only return plain objects
for props, e.g.:

export async function getServerSideProps() {
  return {
    props: {
      text: 'some value',
    }
  }
}

With this commit, the props object can also be a Promise, e.g.

export async function getServerSideProps() {
  return {
    props: (async function () {
      return {
        text: 'promise value',
      }
    })(),
  }
}

For now, the framework simply waits for the results of the props Promise to resolve,
but this small change sets the groundwork for later allowing props to be streamed (cc @devknoll).

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR. -- This is part of @devknoll's ongoing work to support streaming.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not. not sure if this applies here
• Errors have helpful link attached, see contributing.md

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js serverprops	Change
buildDuration	13.6s	13.7s	⚠️ +196ms
buildDurationCached	3.1s	3.2s	⚠️ +28ms
nodeModulesSize	61.7 MB	61.7 MB	⚠️ +337 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	kara/next.js serverprops	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.454	2.56	⚠️ +0.11
/ avg req/sec	1018.59	976.67	⚠️ -41.92
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.354	1.369	⚠️ +0.01
/error-in-render avg req/sec	1846.19	1826.23	⚠️ -19.96

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js serverprops	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	23.3 kB	23.3 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	67.2 kB	67.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js serverprops	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js serverprops	Change
_app-HASH.js gzip	979 B	979 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.67 kB	2.67 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	918 B	918 B	✓
image-HASH.js gzip	4.14 kB	4.14 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	318 B	318 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	13 kB	13 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js serverprops	Change
_buildManifest.js gzip	491 B	491 B	✓
Overall change	491 B	491 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js serverprops	Change
index.html gzip	540 B	540 B	✓
link.html gzip	553 B	553 B	✓
withRouter.html gzip	534 B	534 B	✓
Overall change	1.63 kB	1.63 kB	✓

---

Webpack 4 Mode (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js serverprops	Change
buildDuration	11.7s	11.4s	-311ms
buildDurationCached	4.8s	4.7s	-57ms
nodeModulesSize	61.7 MB	61.7 MB	⚠️ +337 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	kara/next.js serverprops	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.49	2.644	⚠️ +0.15
/ avg req/sec	1003.83	945.71	⚠️ -58.12
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.373	1.426	⚠️ +0.05
/error-in-render avg req/sec	1821.02	1752.84	⚠️ -68.18

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js serverprops	Change
16.HASH.js gzip	186 B	186 B	✓
677f882d2ed8..HASH.js gzip	14.1 kB	14.1 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	10.7 kB	10.7 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	68.1 kB	68.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js serverprops	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js serverprops	Change
_app-HASH.js gzip	964 B	964 B	✓
_error-HASH.js gzip	3.8 kB	3.8 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.87 kB	2.87 kB	✓
head-HASH.js gzip	3.06 kB	3.06 kB	✓
hooks-HASH.js gzip	924 B	924 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	3.03 kB	3.03 kB	✓
withRouter-HASH.js gzip	295 B	295 B	✓
30809af5c834..565.css gzip	125 B	125 B	✓
Overall change	18.1 kB	18.1 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js serverprops	Change
_buildManifest.js gzip	500 B	500 B	✓
Overall change	500 B	500 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js serverprops	Change
index.html gzip	584 B	584 B	✓
link.html gzip	597 B	597 B	✓
withRouter.html gzip	578 B	578 B	✓
Overall change	1.76 kB	1.76 kB	✓

Commit: 376a2f4