http: validate headers in `writeEarlyHints` #3597

Job	Run time
Build slim tarball	2m 30s
x86_64-linux: with shared libraries	21m 44s
x86_64-darwin: with shared libraries	28m 27s
aarch64-linux: with shared libraries	19m 27s
aarch64-darwin: with shared libraries	20m 53s
	1h 33m 1s

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