scalapb-runtime-grpc: optimize Marshaller for InProcessTransport #1966

Job	Run time
conformance	1m 37s
build (2_12, mima)	1m 16s
build (2_12, e2e)	5s
build (2_12, test)	3m 7s
build (2_13, mima)	1m 21s
build (2_13, e2e)	6s
build (2_13, test)	3m 4s
build (3, mima)	3s
build (3, e2e)	6s
build (3, test)	3m 4s
build (2_12, examples_and_formatting)	2m 49s
build (2_13, scala3_compat_test)	3m 5s
build (3, scala3_compat_test)	3m 4s
ci-passed	0s
	22m 47s

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