grpc: Add interceptor API

[Jozott00]

Subsystem
gRPC

Problem Description
A main feature of all gRPC libraries is their interceptor API.
We want a Kotlin idiomatic simple-to-use interceptor API (for client and server) that allows users to implement things like authentication, logging or tracing.

Solution
This PR adds a convenient Kotlin idiomatic interceptor API.

E.g. a (over-simplified) server-side authorization interceptor could look like this

val myAuthInterceptor = object : ServerInterceptor {
    override fun <Request, Response> ServerCallScope<Request, Response>.intercept(request: Flow<Request>): Flow<Response> =
        flow {
            val authorized = mySuspendAuth(requestHeaders)
            if (!authorized) {
                close(Status(StatusCode.PERMISSION_DENIED, "Not authorized"))
            }

            proceedUnmodified(request)
        }
     }

[Mr3zee]

Great work! I have some comments nitpicking some details, mostly DSL, but everything else is well designed

and btw kDocs are really good there, quite easy to read and understand

[Mr3zee]

Great, thank you!

[morki]

Will there be an interceptor feature in KRPC too? It would be really handy for the same use cases as presented here.

[Mr3zee]

@morki in some sort, yes, though later, we only maintain kRPC now, until we finish with gRPC (which is not much else to do, but still will take a couple of months)