Let CodeGenerators provide extension for CustomOptions. #1457

Job	Run time
Api Breakage Compared to main branch	1m 40s
fuzzing_regressions (debug)	3m 24s
core (5.10.1-noble, ref/heads/main)	16m 9s
fuzzing_regressions (release)	2m 55s
sanitizer_testing (address, debug)	2m 34s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	13m 54s
sanitizer_testing (address, release)	4m 57s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	13m 3s
sanitizer_testing (thread, debug)	2m 38s
sanitizer_testing (thread, release)	4m 28s
	1h 5m 42s

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