feat(DataResolver): prefer streams over buffers

[lilliliilil]

Please describe the changes this PR makes and why it should be merged:

I've noticed high memory usage when tried to send a few ~100mb files as message attachments so I went to the source code and discovered that, indeed, everything is being converted to Buffers for some reason. Since discord.js uses node-fetch under the hood, which accepts Streams, the fix seems to be pretty simple.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[lilliliilil]

Uh, don't merge, getting DiscordAPIError: Cannot send an empty message when trying to send some mp4 by url.

[lilliliilil]

Some progress on the DiscordAPIError: Cannot send an empty message issue:

const https = require('https');
const FormData = require('form-data');
const fetch = require('node-fetch');

const httpbin = async (key, value) => {
  const body = new FormData();
  body.append(key, value);
  const res = await fetch('http://httpbin.org/post', { body, method: 'POST' });
  console.log(await res.json());
};

(async () => {
  const url = 'https://discord.js.org/index.html';

  // works, Content-Length header is empty
  await httpbin('index.html', await new Promise(ok => https.get(url, ok)));

  // doesn't work, Content-Length is being set to some numeric value
  await httpbin('index.html', await fetch(url).then(r => r.body));
})();

[lilliliilil]

Alright, found it: form-data/form-data#382 fixes the issue I'm having here in this PR. Unfortunately, it doesn't seem like it will be merged in near future.

In a nutshell: there's a bug in form-data so it can't handle all streams in the same way. Streams returned by fs.createReadStream are handled fine, as well as streams returned by http.get and https.get, but other than these, no streams can be considered safe to use with the current version of form-data.