TEP-0090: Looping [Problem Statement]

This change adds the problem statement for Looping. It scopes the
problem, describes the use cases, and identifies the goals, the
requirements for the solution, and related work in other continuous
delivery systems.

Today, users cannot supply varying `Parameters` to the same `Task` or
`Custom Task` - that is, fan out their `Task` or `Custom Tasks`.

In this TEP, we aim to provide a way to run the same `Task` or
`Custom Task` with varying `Parameters` by spinning up a `TaskRun` or
`Run` for each `Parameter` in a loop.

This looping construct is aimed at improving the composability,
scalability, flexibility and reusability of *Tekton Pipelines*.

References:
- [Task Loops Experimental Project][task-loops]
- Issues:
  - tektoncd/pipeline#2050
  - tektoncd/pipeline#4097

[task-loops]: https://github.com/tektoncd/experimental/tree/main/task-loops