fix: pass pull request header and footer options to merge plugin

[lukekarrys]

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Fixes #2142 🦕

Details

This works by looping through repositoryConfig and getting the first set value for both pullRequestHeader and pullRequestFooter. This means they could possibly come from different components in a manifest release.

Alternate Solutions

I thought about only using config values from repositoryConfig[ROOT_PROJECT_PATH]. I didn't have strong feelings either way so I went with the solution I felt would be less surprising. But I'd be open to reworking the PR to this alternate solution.

[chingor13]

Thanks for this! The feature seems reasonable, but this one has a bit more complex logic and could use a unit test to demonstrate the example and to ensure we don't regress in the future.

[lukekarrys]

Thanks @chingor13! I added a test case where it grabs the header from the first package and the footer from the second package another, while also ignoring the footer from the second package. Let me know what you think.

[lukekarrys]

I didn't realize I left the tests in a failing state. Just pushed a fixup that should be passing now.

[chingor13]

Thanks!