feat: add configurable span context extractor #18

Job	Run time
build (macos-latest, stable)	18s
build (windows-latest, stable)	2m 27s
build (ubuntu-latest, nightly)	1m 5s
build (macos-latest, 1.83.0)	1m 20s
build (macos-latest, nightly)	1m 8s
build (windows-latest, nightly)	2m 36s
build (ubuntu-latest, stable)	26s
build (ubuntu-latest, 1.83.0)	1m 10s
build (windows-latest, 1.83.0)	1m 5s
	11m 35s

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