scalapb-runtime-grpc: optimize Marshaller for InProcessTransport #1969

Job	Run time
conformance	1m 32s
build (2_12, mima)	2m 48s
build (2_12, e2e)	8s
build (2_12, test)	13m 23s
build (2_13, mima)	1m 20s
build (2_13, e2e)	8s
build (2_13, test)	10m 26s
build (3, mima)	8s
build (3, e2e)	7s
build (3, test)	8m 54s
build (2_12, examples_and_formatting)	2m 58s
build (2_13, scala3_compat_test)	7m 12s
build (3, scala3_compat_test)	8m 30s
ci-passed	1s
	57m 35s

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