feat(services/github): add github contents support #214

Job	Run time
build_all_features	5m 16s
build_all_platforms (ubuntu-latest)	2m 48s
build_default_features (ubuntu-latest)	1m 9s
build_all_platforms (macos-latest)	4m 20s
check_clippy	4m 11s
build_default_features (macos-latest)	1m 35s
build_under_wasm	49s
build_all_platforms (windows-latest)	5m 24s
build_default_features (windows-latest)	2m 53s
check_docs	3m 29s
unit	3m 27s
check_msrv	4m 1s
	39m 22s

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