feat(impl_jni): add JCA RSA-PSS implementation

[mfazrinizar]

Summary

Adds the JNI/JCA RSA-PSS implementation on top of the Android JCA backend and the persistent RSA key ownership introduced in #323.

This PR implements:

• RSA key generation with public exponents 3 and 65537
• PKCS#8 private-key and SPKI public-key import/export
• RSA-PSS JWK import/export
• byte and stream signing and verification
• explicit digest, MGF1 digest, salt-length, and trailer-field parameters
• standard RSASSA-PSS provider lookup with hash-specific Android aliases
• focused desktop JNI tests and an Android smoke test covering all supported hashes

Persistent JCA keys are retained through the shared isolate-unsendable key
owner. JNI arenas are used only for temporary values created by an operation.

Testing

Desktop JNI setup, if it has not been run already:

dart run jni:setup

Verified:

dart format --output=none --set-exit-if-changed .
dart analyze
dart test test/impl_jni_rsapss_test.dart
dart test test/webcrypto_test.dart -p vm -n RSA-PSS
cd example/webcrypto_demo_flutter_app
flutter test integration_test/jni_rsapss_test.dart -d emulator-name
git diff --check

[HamdaanAliQuatil]

nit: zero is a valid rsa-pss salt length, including in our shared vectors, so this should say must be a non-negative integer

[mfazrinizar]

Thanks. Updated the message to must be a non-negative integer.

[HamdaanAliQuatil]

non-blocking follow-up: please record RSA-PSS as a provider/API-level limitation. Android’s hash-specific SHAxxxwithRSA/PSS aliases begin at API 23, while the generic RSASSA-PSS entry is not available on API 9-22. so API 21-22 support is provider-dependent and cannot be guaranteed by this implementation alone.

see https://developer.android.com/reference/java/security/Signature

[mfazrinizar]

non-blocking follow-up: please record RSA-PSS as a provider/API-level limitation. Android’s hash-specific SHAxxxwithRSA/PSS aliases begin at API 23, while the generic RSASSA-PSS entry is not available on API 9-22. so API 21-22 support is provider-dependent and cannot be guaranteed by this implementation alone.

see https://developer.android.com/reference/java/security/Signature

Noted. Recorded this as an inline TODO next to the RSA-PSS provider selection. It documents API 21–22 as provider-dependent and leaves the eventual policy open: document limited support, require API 23, or route unsupported cases through another backend.