Expose default merge strategy through the REST API

Issue BCLOUD-13895 added a default merge strategy for a repository, but it has to be configured manually on all repositories. That's not a scalable solution; it would be nice to be able to configure the merge strategy for existing repositories, and (optionally/ideally) to set it at the time a repository is created.

(Please also consider making all new administration UI features part of the REST API, e.g. BCLOUD-16227.)