scalapb-runtime-grpc: optimize Marshaller for InProcessTransport #1964

Job	Run time
conformance	1m 51s
build (2_12, mima)	1m 16s
build (2_12, e2e)	6s
build (2_12, test)	1m 27s
build (2_13, mima)	1m 9s
build (2_13, e2e)	4s
build (2_13, test)	1m 23s
build (3, mima)	4s
build (3, e2e)	8s
build (3, test)	1m 25s
build (2_12, examples_and_formatting)	1m 26s
build (2_13, scala3_compat_test)	1m 11s
build (3, scala3_compat_test)	1m 34s
ci-passed	0s
	13m 4s

Provide feedback