JavaKit: add CustomJavaClassLoaderConstructible protocol #275

Job	Run time
Swift tests (swift:nightly-main jdk:Corretto os:jammy)	5m 43s
Java tests (swift:nightly-main jdk:Corretto os:jammy)	5m 54s
soundness / Unacceptable language check	3s
soundness / Documentation check	38s
soundness / License headers check	7s
soundness / Format check	0s
soundness / API breakage check	0s
soundness / Broken symlinks check	2s
soundness / Shell check	36s
soundness / YAML lint check	3s
soundness / Python lint check	5s
	13m 11s

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