Add optional support for diff_cover reports

[jorgeAtViboo]

• Adds an optional argument diffCoverageFile to the action which is a report from https://github.com/Bachmann1234/diff_cover which offers a more detailed list of line changed in the diff file as opposed to the modified file in it's entirety. The goal being to only evaluate the changes made and not get punished for making changes in a large untested file.
• Built files are also included in the original repo and serve for the test.yml action. You can forget about them for the review.
• The comment below is generated by said action with the base being the example coverage files coverage-diff.json and coverage.xml.

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
1000	940	94%	0%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
src/coverage.ts	69%	🟢
TOTAL	69%	🟢

updated for commit: ff6ed38 by action🐍