[GR-60640] Expose CapnProtoSchemaHolder to ImageLayerSingletons. #20721

Job	Run time
/substratevm on Windows	8m 26s
/compiler style,fullbuild,test JDK21	31m 45s
/compiler build,bootstraplite JDKlatest	4m 15s
/espresso style,fullbuild JDK21	13m 29s
/substratevm style,fullbuild JDK21	11m 15s
/substratevm build,helloworld,native_unittests JDKlatest	20m 37s
/substratevm build,debuginfotest JDKlatest	22m 20s
/substratevm hellomodule JDKlatest	4m 54s
/sulong style,fullbuild,sulongBasic JDK21	20m 15s
/truffle JDK21	34m 34s
/vm build,sulong JDKlatest	26m 47s
/vm build JDKlatest	15m 51s
	3h 34m 28s

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