[Feature Request] Implement Branch Context for flags

[nikosatwork]

What product do you want to improve?
Web application, API

Is your feature request related to a problem? Please describe.
We would like to be able to view and filter by flags on all branches of the repository, not just the default branch, in the UI and via the API

Describe the solution you'd like
Similar to the Branch Context drop down in the Components tab in the web application and the branch parameter in the components API, implement these for the Flag tab and flags API respectively

Describe alternatives you've considered
n/a