Add `Headers.get_as_optional_string` and `get_as_string` #2232

Job	Run time
Integration / S3 Express One Zone tests (Ubuntu x86, FUSE 2)	10m 32s
Integration / Tests (Ubuntu x86, FUSE 2)	8m 46s
Integration / Tests (Amazon Linux arm, FUSE 2)	9m 25s
Integration / S3 Express One Zone tests (Amazon Linux arm, FUSE 2)	7m 33s
Integration / S3 Express One Zone tests (Ubuntu x86, FUSE 3)	6m 7s
Integration / Tests (Ubuntu x86, FUSE 3)	11m 46s
Integration / Address sanitizer	6m 36s
	1h 0m 45s

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