DRAFT: Add support for slather's new --ymlfile option

[jarrodlombardo-EventBase]

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

To be able to use slather from fastlane for a repo that has more than one .xcodeproj, slather needs to add support for custom ymlfiles. One that PR is released by Slather, this PR can be updated and merged to support that param from fastlane.

#21612

Description

• Added ymlfile support to the Fastlane.swift slather function.
• Added --ymlfile output to the slather action.
  • only allow ymlfile use for slather 2.8.0+ (This number will need to be changed if the slather release is a different version; latest is currently 2.7.5)
  • when checking for a config file, check the path of the ymlfile if provided and the regular .slather.yml if not.
• added validate_params and ymlfile_available tests to the test spec.
• Updated other tests to include the ymlfile param.

Testing Steps

bundle exec rspec and bundle exec rubocop -a are passing.
In the spec, the it "works with all parameters" test is generating the correct slather commandline with the new param.

[getaaron]