parsec authentication: drop BouncyCastle requirement on Java 8/11 and provider-dependent key derivation on Java 15+

On Java 8/11, parsec authentication requires BouncyCastle for Ed25519 signing.
On Java 15+, the public key is derived through a KeyPairGenerator fed by a SecureRandom override, which depends on the provider consuming exactly one 32-byte nextBytes() as the seed.
The ref10 Ed25519 code already vendored for client_ed25519 computes standard RFC 8032 signatures when given a 32-byte seed (verified byte-identical to the JDK's).
Share it between both plugins: parsec then works on all supported Java versions without extra dependency, and the multi-release variant, PKCS#8 wrapping and JDK/BC branching are removed.