JavaKit: add CustomJavaClassLoaderConstructible protocol #269

Job	Run time
Java tests (swift:nightly-main jdk:Corretto os:jammy)	7m 27s
Swift tests (swift:nightly-main jdk:Corretto os:jammy)	5m 52s
soundness / Documentation check	34s
soundness / Broken symlinks check	5s
soundness / API breakage check	0s
soundness / Unacceptable language check	3s
soundness / License headers check	9s
soundness / Format check	0s
soundness / Python lint check	8s
soundness / Shell check	36s
soundness / YAML lint check	4s
	14m 58s

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