[#263] Implement client for exchanging raw messages using gRPC protocol

Implement gRPC client that can uniformly execute any RPC on the remote
server. In the primary implementation, the client is a thin wrapper over
gRPC client connection that is required to create the client. In the future,
it is planned to expand the library with convenient functionality.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

rpc/grpc/call_options.go

@@ -0,0 +1,25 @@
+package grpc
+
+import (
+	"context"
+)
+
+// CallOption is a messaging session option within RPC.
+type CallOption func(*callParameters)
+
+type callParameters struct {
+	ctx context.Context
+}
+
+func defaultCallParameters() *callParameters {
+	return &callParameters{
+		ctx: context.Background(),
+	}
+}
+
+// WithContext returns option to set RPC context.
+func WithContext(ctx context.Context) CallOption {
+	return func(prm *callParameters) {
+		prm.ctx = ctx
+	}
+}