Promote branch model API to public API

When the branch model was launched, the API was still in flux and we kept the BranchModelConfiguration and BranchModelConfigurationService internal. Plugins cannot retrieve the configuration, update the configuration or enable/disable the branch model programatically.

The branch model is now stable enough for us to promote this to public API. Likewise, we should add the REST endpoints to allow enabling/disabling and configuring the branch model for a repository.