Support for adding license to files whose name has suffix ".p4"

[jafingerhut]

Files whose names have the suffix '.p4" are typically written in the P4 programming language: https://p4.org

[google-cla[bot]]

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[jafingerhut]

@smolkaj @jonathan-dilorenzo These people working at Google work on projects that use the P4 programming language.

[flwyd]

Thanks for contributing to addlicense! Please sync your pull request with master; I've changed filetypes to one-per-line to make future merge conflicts less likely. Please also add a test case in the testdata/initial and testdata/expected directories.

[jafingerhut]

The error messages I see in the log for the failing "linter / lint" CI test appear to have little to do with my 1-line change, which could not have possibly by itself introduced so many linter errors.

[flwyd]

Please add test cases with a before and after file in the testdata directory, and add a case to TestLicenseHeader in main_test.go.

I've fixed the linter errors; another sync with the master branch might be needed to pick up the new workflow definition.