rpc : add support for multiple devices

[rgerganov]

Allow rpc-server to expose multiple devices from a single endpoint. Change RPC protocol to include device identifier where needed. Add new API to get the device count from an RPC endpoint.

closes: #15210

[rgerganov]

@slaren I am still working on this but I'd appreciate some early feedback on the API changes in ggml-rpc.h

[rgerganov]

I am also considering changing the naming scheme for RPC devices. Now I am using RPC<X>[<host>:<port>] for both name and description which doesn't look nice in various logs.

I am thinking of switching to RPC<X> for device name and [<host>:<port>] for device description. The X number should be global monotonic counter making RPC devices appear like this:

RPC0 ([localhost:50052])
RPC1 ([localhost:50052])
RPC2 ([localhost:18053])
RPC3 ([localhost:18054])
...

Thoughts?