Add resolver interface and base implementation [stacked on #311]

[panchr]

This PR is stacked on top of #311.

Summary

A Resolver is responsible for providing the abstraction that multiple watchers are one: it decides how to merge the list of backends from each one. This PRs adds:

• the interface for the Resolver
• a base implementation
• documentation on how to create new Resolvers
• MultiWatcher loads the resolvers

Testing

• [x] Unit tests

Reviewers

@bsherrod @austin-zhu