Add UI components for branch workflows

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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 issue we need to create the necessary UI components for branch workflows on the Branch rules details page.

See the spike for more context.

Note: if the backend endpoint is not available by the time this issue is picked up we should not render anything in the UI yet. We can build the frontend as part of this issue and do the backend/frontend integration in a separate issue.