feature: add support for commit input (fixes #1634)

[testersen]

Fixes #1634

This pull request adds support for supplying a commit input. This is necessary in order to replicate the standard behavior in pull request synchronization events when not using the default values.

Commit	Summary
650ceb0	Adds a commit input in the action.yaml file
67b5caa	In the input-helper file, before settings.ref is checked and assigned default values, set settings.commit to equal the inputs.commit.
8a241b5	In the input-helper file, change settings.ref = core.getInput('ref') to settings.ref = core.getInput('ref') ?? settings.ref as a fallback for when the commit input was specified, but not the ref input.
267ca9c	Before making settings.ref fallback to settings.commit, validate that the settings.commit, if defined, has the format of a full SHA sum.
3a6c8fb	Added tests to verify the behavior of implementation in this pull request.
491fae0	Result of running npm run format
a52fa92 1be0f94	Result of running npm run build
0865c4b	In input-helper, change the settings.ref fallback from using ?? to ||, becuase settings.ref might be an empty string or null, causing ?? to not have the desired effect. Also updated the test to reflect the fallback behavior.

[testersen]

cc @joshmgross - we @TelenorNorgeInternal were hoping to get this merged soon ™️ would you by any chance have time to review sometime before September? Thanks in advance!