feat(services/github): add github contents support #231

Job	Run time
build_all_features	5m 17s
build_all_platforms (ubuntu-latest)	2m 43s
build_all_platforms (macos-latest)	3m 24s
build_all_platforms (windows-latest)	4m 21s
build_default_features (ubuntu-latest)	1m 38s
build_default_features (macos-latest)	1m 43s
build_under_wasm	47s
build_default_features (windows-latest)	2m 8s
check_clippy	3m 56s
check_docs	3m 38s
check_msrv	3m 58s
unit	3m 42s
	37m 15s

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