Implement `TMT_PLAN_SOURCED_FILE` hook

[LecrisUT]

Generate a file for each plan that is sourced at the beginning of each:

• prepare.shell step
• test invocation

These are explicitly not invoked on all internal guest execute in order to minimize side-effects, and instead these are meant to be used by the user for their test environment. This file is thus exposed as TMT_PLAN_SOURCED_FILE and it is a no-op until they populate it via another prepare step or test invocation.

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Pull Request Checklist

• [x] implement the feature
• [x] write the documentation
• [x] extend the test coverage
• [x] mention the version
• [x] include a release note

Closes #3981

[LecrisUT]

Depends on #4312

[psss]

Missing docs, release note and test coverage, some discussions to be finished, moving to the next sprint as agreed.

[psss]

/packit retest-failed

[psss]

/packit build

[psss]

Irrelevant container image signature failures on centos-stream-10, otherwise green, merging.