Add test coverage for AdditionalHeaders in Streamable HTTP transport … #1837

Job	Run time
build (ubuntu-latest, Release)	6m 46s
build (windows-latest, Release)	11m 9s
build (windows-latest, Debug)	11m 6s
build (ubuntu-latest, Debug)	7m 8s
build (macos-latest, Release)	6m 20s
build (macos-latest, Debug)	8m 21s
publish-coverage / publish-coverage	32s
	51m 22s

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