[jextract] add support for strings in JNI #571

Job	Run time
Java tests (swift:6.0.2 jdk:Corretto os:jammy)	10m 15s
Swift tests (swift:6.0.2 jdk:Corretto os:jammy)	5m 2s
Verify Sample JavaDependencySampleApp (swift:6.0.2 jdk:Corretto os:jammy)	4m 51s
Verify Sample JavaKitSampleApp (swift:6.0.2 jdk:Corretto os:jammy)	4m 18s
Verify Sample JavaSieve (swift:6.0.2 jdk:Corretto os:jammy)	1m 0s
Verify Sample SwiftAndJavaJarSampleLib (swift:6.0.2 jdk:Corretto os:jammy)	6m 26s
Verify Sample JavaProbablyPrime (swift:6.0.2 jdk:Corretto os:jammy)	4m 8s
Verify Sample SwiftKitSampleApp (swift:6.0.2 jdk:Corretto os:jammy)	6m 38s
soundness / Documentation check	37s
soundness / Unacceptable language check	4s
soundness / Python lint check	5s
soundness / Shell check	35s
soundness / API breakage check	0s
soundness / License headers check	5s
soundness / YAML lint check	6s
soundness / Format check	0s
soundness / Broken symlinks check	5s
	44m 15s

Provide feedback