Support releasing Testflight builds to specific beta groups

[aaronsky]

Is your feature request related to a problem? Please describe.

In some cases I want to deliver an individual build to a specific beta group or beta tester without changing the entire configuration, as a one-off.

Describe the solution you'd like

• specify the specific build to release
• specify the beta groups or testers to release to
• support doing so without modifying the config, making these flags take precedence

Describe alternatives you've considered

• considered a new yaml to use for smaller overrides. too complicated.

Additional context

• useful for individual teams who want to deploy to testflight in limited scenarios, rather than wide rollouts.