Add support for setting Coursier's --extra-jars option when launching Scala Steward

[lgmyrek]

copy of #467

tested with released version from my fork, successfully integrated with google cloud artifact registry with:

      - name: Fetch gar-coursier
        id: fetch_gar_coursier
        run: |
          export GAR_COURSIER_CLASSPATH=$(cs fetch --classpath "dev.rolang:gar-coursier_2.13:0.1.3");
          echo "GAR_COURSIER_CLASSPATH=$GAR_COURSIER_CLASSPATH" >> $GITHUB_ENV;
          
      - name: Launch Scala Steward
        uses: lgmyrek/scala-steward-action@v2
        with:
          ...
          extra-jars: ${{ env.GAR_COURSIER_CLASSPATH }}
          cache-ttl: 0minute

[github-actions]

A snapshot release has been created as snapshots/579.

You can test it out with:

uses: scala-steward-org/scala-steward-action@snapshots/579

It will be automatically recreated on any change to this PR.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
core	100%	100%	0	✔
modules	69%	88%	0	➖
Summary	70% (475 / 677)	89% (73 / 82)	0	➖

[lgmyrek]

@alejandrohdezma just like I mentioned in the pr description, I've already tested it from released version from my fork.
Would be great to have it part of official action. Is there anything you want me to do to make that happen?

[alejandrohdezma]

Nope! Was just waiting for CI to pass 😸

Thank you very much!