ServerSnapshot: Improve API and allow filtering with custom struct enumerator

[mgravell]

so we aren't limited just to ReadOnlySpan<ServerEndpoint>, and don't need the allocatey ToArray()

implemented as custom iterator which allows async and LINQ to work directly' existing code still uses span for efficiency, with the new API used in limited scenarios only

intent here:

1. provide an efficient basis for filtered "all server's matching X" functionality
2. provide the Where(CommandFlags) basis for upcoming broadcast work
3. avoid some unnecessary allocations

[NickCraver]

This looks awesome - no notes, much cleaner and dig the implementation. Good call going this route!