Move prost codegen to outdir #1047

Job	Run time
check-vegafusion-python	10m 39s
test-rust (ubuntu-20.04)	21m 9s
test-rust (windows-2022)	36m 15s
test-rust (macos-14)	14m 43s
build-vegafusion-python-osx-arm64	18m 15s
build-vegafusion-python-linux-arm64	15m 45s
build-vegafusion-python-linux-64	14m 19s
build-vegafusion-python-osx-64	34m 18s
build-vegafusion-python-win-64	32m 42s
build-vegafusion-wasm	5m 4s
check-rust	5m 59s
build-vegafusion-server-linux-64	9m 49s
build-vegafusion-server-linux-arm64	8m 42s
build-vegafusion-server-osx-64	33m 42s
build-vegafusion-server-osx-arm64	9m 43s
build-vegafusion-server-win-64	32m 10s
test-vegafusion-python-linux-64	26m 5s
test-vegafusion-python-win-64	1m 55s
bundle-vegafusion-server-builds	7s
test-vegafusion-python-osx-arm64	1m 7s
bundle-vegafusion-python-wheels	9s
	5h 32m 37s

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