[GR-60640] Expose CapnProtoSchemaHolder to ImageLayerSingletons. #20716

Job	Run time
/substratevm on Windows	8m 7s
/compiler style,fullbuild,test JDK21	32m 46s
/compiler build,bootstraplite JDKlatest	4m 18s
/espresso style,fullbuild JDK21	13m 20s
/substratevm style,fullbuild JDK21	11m 31s
/substratevm build,helloworld,native_unittests JDKlatest	20m 48s
/substratevm build,debuginfotest JDKlatest	23m 21s
/substratevm hellomodule JDKlatest	5m 3s
/sulong style,fullbuild,sulongBasic JDK21	20m 59s
/truffle JDK21	34m 31s
/vm build,sulong JDKlatest	27m 15s
/vm build JDKlatest	15m 42s
	3h 37m 41s

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