Add support for Application Credential authentication #254

Job	Run time
Documentation	2m 2s
Test on stable ()	1m 28s
Test on stable (--no-default-features)	1m 26s
Formatting	8s
Test on stable (--no-default-features --features default-rustls)	1m 31s
Test derive on stable	59s
Test on nightly ()	1m 42s
Test derive on nightly	1m 11s
Test derive on 1.64.0	4m 14s
Test on nightly (--no-default-features)	1m 39s
Test on nightly (--no-default-features --features default-rustls)	1m 37s
Test on 1.64.0 ()	6m 13s
Test on 1.64.0 (--no-default-features)	4m 37s
Test on 1.64.0 (--no-default-features --features default-rustls)	5m 51s
	34m 38s

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