[action][git_add] Add force option

[tomorrowkey]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

I want to commit a ignored file in CI.

Description

Add force option to git_add action.
The default value of the action is false.

Testing Steps

[lacostej]

LGTM.

Thanks for the contribution and the test!