Create an endpoint for branch workflows (query + mutation)

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Summary

We want to move the target branch workflows feature into branch rules to better support consolidation of these settings.

As part of this effort, we need an endpoint for managing branch workflows on the branch rule details page (either use the existing branch rule details endpoint, or a separate one, whatever makes the most sense)

• (branch workflow controller) - ~/controllers/projects/target_branch_rules_controller.rb