feat: Add PS256 Support

[tanya732]

Summary

• Adds Algorithm.RSA256PSS, RSA384PSS, and RSA512PSS factory methods (each with RSAKeyProvider, RSAPublicKey + RSAPrivateKey, and RSAKey overloads), implementing the RSASSA-PSS signature scheme from RFC 7518 §3.5. Closes Support for PS256 algorithm #659.

• Non-breaking, purely additive. Existing RS/ES/HS algorithms are unchanged, and the japicmp apiDiff check reports no breaking API changes.

• Threads an optional AlgorithmParameterSpec through CryptoHelper and RSAAlgorithm. For PSS, this is a PSSParameterSpec configured with:

  • MGF1 using the same digest algorithm
  • Salt length equal to the digest length (32/48/64 bytes)
  • Trailer field 1

  The parameters are applied via Signature.setParameter() during both signing and verification. Existing algorithms continue to pass null, so their behavior is unchanged.

Provider / Java Version Notes

• Uses the standard JCA algorithm name RSASSA-PSS, resolved through Signature.getInstance(...) using the first registered provider that supports it. No provider-specific detection logic is added.
• Java 11+
  • RSASSA-PSS is supported natively by the built-in SunRsaSign provider (JDK-8146293).
• Java 8
  • The built-in JDK providers do not implement RSASSA-PSS.
  • Users must register a provider that supports it (for example, Bouncy Castle).
  • No new runtime (implementation) dependency has been added.
  • Bouncy Castle remains a testImplementation dependency only.

Naming

The new factory methods follow the existing naming convention:

• RSA256PSS
• RSA384PSS
• RSA512PSS

The generated JWT alg header values remain the standard JWS values:

• PS256
• PS384
• PS512

Test Plan

• Unit tests pass on Java 8, 11, 17, and 21 (testJava8, testJava11, testJava17, testJava21)
• apiDiff (japicmp) confirms there are no breaking public API changes
• RSAPSSAlgorithmTest covers:
  • PS256/PS384/PS512 sign/verify round-trips
  • Sign with private key / verify with public key
  • Non-deterministic signatures (randomized salt)
  • Invalid public key rejection
  • Mismatched algorithm rejection (e.g. PS256 token verified with PS384/PS512)
  • Native SunRsaSign provider path (skipped on Java 8 using Assume)
• README updated with:
  • PS256/PS384/PS512 algorithm table entries
  • Java 8 provider requirement note