Add RunReason and CustomRunReason

[jerop]

Changes

Prior to this change, we did not have RunReason and CustomRunReason. As a result, there were instances where we reused TaskRunReason and PipelineRunReason. This worked because the underlying strings are the same, but it is confusing when reading the code (and reviewing changes).

In this change, we:

• introduce RunReason and CustomRunReason
• clean up existing reasons to use the enum
• add three common reasons - started, running, successful and failed

/kind cleanup

Follow up to my comments in #5662 (comment) and #5662 (comment) (cc @abayer)

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [n/a] Has Docs included if any changes are user facing
• [n/a] Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [n/a] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [n/a] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/run_types.go	59.3%	57.1%	-2.1
pkg/apis/pipeline/v1beta1/customrun_types.go	59.3%	57.1%	-2.1

[abayer]

fwiw, compilation failure can be found at https://prow.tekton.dev/view/gs/tekton-prow/pr-logs/pull/tektoncd_pipeline/5718/pull-tekton-pipeline-build-tests/1588255157798309888#1:build-log.txt%3A1369-1373

[abayer]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abayer

The full list of commands accepted by this bot can be found here.

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• OWNERS [abayer]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1alpha1/run_types.go	59.3%	57.1%	-2.1
pkg/apis/pipeline/v1beta1/customrun_types.go	59.3%	57.1%	-2.1

[tekton-robot]

The following Tekton test failed:

Test name	Commit	Details	Required	Rerun command
check-pr-has-kind-label	a2044ac	link	true	/test check-pr-has-kind-label

[lbernick]

/lgtm

[jerop]

events flake

/test tekton-pipeline-unit-tests