Depends on #5480

Context

With the default channel configuration moving to the configuration repo, we should make it so darc add-default-channel can modifies the repo config. During the transition period, this should be behind a flag in the command.

Goal

Modify the existing darc add-default-channel command so it adds the default-channel by modifying the configuration repo in whichever structure we agree on, using the available yaml models. We'll need to add a few flags to the command:

• Flag to target the repo instead of the API
• Flag to specify configuration repository
• Flag to specify which branch to commit the config changes to (if we already have a PR, or for scenario tests)
• Flag to specify which base branch to use for the PR creation
• Flag to specify we don't want to open a PR (for scenario tests)
• Flag to overwrite default file for the default channel, and put the new default channel in the provided file

This should be written so it makes it easy to remove the API part, and use the configuration repo as one and only way to modify the configuration