Conditional reader for azblob, gcs, oss #2336

Job	Run time
build_all_features	4m 56s
build_all_platforms (ubuntu-latest)	2m 33s
build_default_features (ubuntu-latest)	42s
build_all_platforms (macos-latest)	2m 44s
build_default_features (macos-latest)	1m 11s
build_under_wasm	57s
build_all_platforms (windows-latest)	4m 55s
build_default_features (windows-latest)	1m 53s
check_clippy	3m 38s
check_docs	3m 26s
check_msrv	37s
unit	7m 2s
	34m 34s

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