Add automatic registry mux refreshing

[jclapis]

This is part of #382 and incorporates some additional user feedback. Basically, users want the PBS service to routinely query SSV or Lido when set up with muxes that use them as registries, and automatically support new pubkeys that get added to the corresponding operators. This PR adds that capability by:

• Identifying the relevant registry muxes upon validation
• Detecting on startup whether or not registry refreshing is required
• Running a parallel task to the PBS service that occasionally locks the state, queries the registries, and adds any new pubkeys to the config's pubkey -> mux config HashMap.

Note this has a substantial rewrite of the SSV loader, including breaking the core structs out into a public module and a more robust mock of the SSV API server for testing.