jextract: Find ways to minimize string copying when passing Java String to Swift

[ktoso]

We could consider if we never escape the Swift string or maybe also look into https://download.java.net/java/early_access/jdk24/docs/api/java.base/java/lang/foreign/Linker.Option.html#critical(boolean) if for some opt-in methods we could avoid copying.

We may also consider another JavaString type which would implement StringProtocol and we would avoid creating a real String in the Swift side which today forces another copy.