[remote-resolution] Proof-of-Concept Controller

[ghost]

Changes

This commit adds a controller, several reconcilers, and a set of helpers that
all work in tandem to provide a framework for resolving Tekton resources from
places like git repos. For this proof of concept only Tekton's Pipeline type is
resolvable but in principle the solutions included here are agnostic to the
content being requested.

In more concrete terms this project will take a PipelineRun that looks like
this:

kind: PipelineRun
metadata:
  annotations:
    resolution.tekton.dev/resolver: git
    git.repo: https://github.com/sbwsg/experimental.git
    git.commit: 326ad1a1432b2144fb50f2bfd56875181a407c6b
    git.path: /remote-resolution/pipeline.yaml
spec:
  status: PipelineRunPending

and turn it in to one that looks like this:

kind: PipelineRun
metadata:
  # <same as above>
status:
  # this pipelineSpec has been pulled in from git
  pipelineSpec:
    tasks:
    - name: foo
      taskSpec:
        metadata: {}
        spec: null
        steps:
        - # ... etc ...

Here's a high level overview of the different parts in play:

1. (Shown in the YAML above) PipelineRuns get a new syntax to indicate the
   place that a pipelineSpec should be fetched from. In this proof of concept
   that syntax takes the form of some specific annotations.

2. A component interprets the syntax from (1) and converts it into a generic
   request for a resource. In this proof of concept the "generic request" is
   either an HTTP call or an instance of a new CRD called a ResourceRequest.
   Both have been implemented to try and figure out which approach better suits
   the project's needs.

3. A resolver sees the request from (2) and figures out if it can respond. The
   resolver fetches the resource if so. In this proof of concept three resolvers
   have been implemented: a "noop" resolver for testing, an "in-cluster" resolver
   that can fetch pipelines from inside the cluster, and a "git" resolver that can
   fetch from public git repos.

The idea for (3) is to make writing a Resolver easy and to do so in a way that's
independent of the request format in (2). A resolver shouldn't care if the request
is coming in via HTTP or a CRD or anything else. To that end a go interface has
been introduced in the pkg/reconciler/framework directory that resolvers can
implement to hide all the boilerplate.

See remote-resolution/README.md for more details on installing and trying out
this controller.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes docs (if user facing)
• Commit messages follow commit message best practices
• Commit messages includes a project tag in the subject - e.g. [OCI], [hub], [results], [task-loops]

[tekton-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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You can assign the PR to them by writing /assign @bobcatfish in a comment when ready.

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[tekton-robot]

The following users are mentioned in OWNERS file(s) but are not members of the tektoncd org.

• productivity-writers
  • remote-resolution/hack/OWNERS
  • remote-resolution/hack/OWNERS

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[ghost]

/close

[tekton-robot]

@sbwsg: Closed this PR.

In response to this:

/close

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