refactor: split off `Fork/ForkSource` from `ForkStorage` #1515

Job	Run time
build (ubuntu-latest)	7m 37s
lint	2m 4s
build (macos-latest)	7m 37s
spec-tests / spec	2m 22s
e2e-tests-rust / e2e-rust (ubuntu-latest)	3m 41s
e2e-tests / e2e (ubuntu-latest)	57s
e2e-tests-rust / e2e-rust (macos-latest)	3m 29s
e2e-tests / e2e (macos-latest)	1m 25s
	29m 12s

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