split decoder into spectrogram and vocoder without changing API #2978

Job	Run time
actionlint	22s
rust-lint	4m 12s
shellcheck	13s
validate-cargo-lock	45s
build-and-test-java-api (windows-latest)	6m 37s
build-and-test-java-api (macos-latest)	3m 21s
build-and-test-java-api (ubuntu-latest)	3m 5s
build-and-test-python-api (windows-latest)	6m 58s
build-and-test-python-api (macos-latest)	4m 16s
build-and-test-python-api (ubuntu-latest)	3m 4s
build-unix-cpp-example (macos-latest, osx-x64-cpu-cpp-shared)	1m 50s
build-unix-cpp-example (ubuntu-latest, linux-x64-cpu-cpp-shared)	1m 37s
build-windows-cpp-example	3m 59s
c-header	47s
rust-integration-test-strategy-matrix	1s
rust-unit-test	3m 43s
rust-integration-test (macos-12)	2m 28s
rust-integration-test (ubuntu-20.04)	1m 41s
	48m 59s

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