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 haven't
defined a process to promote them beyond *experimental*.

As such:
- users can't depend on the `Custom Tasks` because they not *stable* (they can change any time).
- contributors don't have a process to stabilize their `Custom Tasks` or integrate them natively
to the Tekton Pipelines API.

In this TEP, we aim to:
- define the graduation path for `Custom Tasks` from *experimental* to *stable* to *integrated*.
- provide infrastructure for *stable* `Custom Tasks` that are officially supported by Tekton.

`Custom Tasks` will have three 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. *Integrated*: `Custom Tasks`' functionalities are natively supported in Tekton Pipelines API.

In this change, we discuss the motivation (goals, non-goals, requirements, use cases), the proposal, the
design evaluation, the alternatives and infrastructure needed.

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