Add gRPC support

[Johni0702]

This PR adds support for connecting to Murmur via gRPC instead of Ice.

Almost all of the Murmur Ice API is bridged to gRPC and existing modules do not need to be changed (and neither do future ones, they should continue to use the Ice API).

Modules that rely on specific exception being thrown by Ice might have a hard time because gRPC exceptions are mostly passed through the bridge instead of being transformed into their Ice counterparts. I'm not aware of any modules actually doing that though.

Tested with idlemove, onjoin, samplecontext, seen, (partially) source, setstatus and gmod.
Ice methods not used by any of these have been tested in the REPL but their return values might not be exactly what modules expect (I merely compared them to the docs, not to ones from an actual Ice client).

[hacst]

Hi. Cool patch. As it's pretty much an independent code-path I see no trouble landing this. Only thing I think needs a bit more love is the readme changes. Would be great if the setup section made clearer that grpc is an optional, "best-effort" alternative to Ice in mumo. As it is now I think some people might be confused about the difference. Maybe something in the direction of: "Besides Ice murmur also supports the alternative gRPC protocol. If using Ice is problematic in your environment gRPC offers you another way to access some (but not all) of mumo's functionality.". Bonus points for a gRPC restrictions section at the end with a short summary of what works and what not with a call to action in case someone wants to chip in to improve it.

Thanks again for the PR!

[raku-cat]

This PR didn't actually work for me using python2.7(python3 didn't work at all, not unexpected, didn't try 2to3 though).
I had to move initdata.logger = CustomLogger() below the actual CustomLogger function else I got an UnboundLocalError.