feature: option to prevent merging PR's with decreased coverage

[carlosdevpereira]

Description

Whenever a pull-request will lower the coverage of the base branch, it would be nice to have a check that prevents those pull requests from being merged. This behaviour can also be a "opt-in" feature, toggled through an input variable passed to the action.

Proposed solution

Add an input variable called prevent-decreased-coverage to the action yml config, set as false by default that, when enabled, compares the coverage of the current commit against the previous one and, in case the coverage is lowered, fails the check. This check should be implemented for both commit history view and also in available pull request checks.