TEP-0087: Custom Tasks Graduation

This change adds the Tekton Enhancement Proposal for `Custom Tasks` Graduation.

Today, `Custom Tasks` shared by the Tekton community are all *experimental*, and we don't have a process to promote
them beyond *experimental*.

As such:
- Users can't depend on the `Custom Tasks` because they can change any time, they may not reflect the Tekton roadmap and
  they may not meet the Tekton standards because they haven't been reviewed in a Tekton Enhancement Proposal.
- Contributors don't have a process to stabilize their `Custom Tasks` or integrate them to the *Tekton Pipelines API*.

In this TEP, we aim to:
- Define the graduation requirements path for `Custom Tasks`.
- Provide `Custom Tasks` that are officially supported by Tekton and have stability guarantees.

`Custom Tasks` will have four stability levels in their graduation path:

1. *Experimental*: `Custom Tasks` can change any time, [*alpha* API policy][api-policy] applies, as we are iterating on
them.
2. *Stable*: `Custom Tasks` are stable, [*beta* API policy][api-policy] applies, and have been approved in a TEP.
3. *Packaged*: `Custom Tasks` are shipped with *Tekton Pipelines* releases, so are available for use out of the box.
4. *Integrated*: `Custom Tasks`' functionalities are natively supported in the *Tekton Pipelines API*.

The *Tekton Pipelines* owners have the discretion to expedite the graduation process of a given `Custom Task`, such as
when they agree early on that they want to integrate the functionality provided by the `Custom Task` directly to the
*Tekton Pipelines API*.

See also [TEP-0002: Enable Custom Tasks](https://github.com/tektoncd/community/blob/main/teps/0002-custom-tasks.md).

[api-policy]: https://github.com/tektoncd/pipeline/blob/main/api_compatibility_policy.md#alpha-beta-and-ga