[spaceship] add get_beta_app_tester_detail

[mlch911]

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

Add the connect api of iris/v1/apps/#{app_id}/betaAppTesterDetail.
Through this API, you can know how many new testers you can still add, and even automate the removal and addition of testers.

Description

Add the connect api of iris/v1/apps/#{app_id}/betaAppTesterDetail.
Through this API, you can know how many new testers you can still add, and even automate the removal and addition of testers.

Testing Steps

[mlch911]

The lint failed with this. But other functions in that file also named prefix with get_.

Offenses:

spaceship/lib/spaceship/connect_api/models/app.rb:390:11: C: Naming/AccessorMethodName: Do not prefix reader method names with get_.
      def get_beta_app_tester_detail
          ^^^^^^^^^^^^^^^^^^^^^^^^^^