Fix key export for non-CRT RSA keys and add Ed25519 PEM support

[nindanaoto]

Fixes connectbot/connectbot#1812

This PR fixes SSH key export failures that occur when using security providers like Conscrypt, which return RSAPrivateKey impleme
ntations that don't implement RSAPrivateCrtKey (e.g., OpenSSLRSAPrivateKey). It also adds Ed25519 support to PEMEncoder.

Problem

When using the OSS build of ConnectBot (which uses Conscrypt's OpenSSL provider), exporting RSA keys fails with "Unsupported key
type" error because:

• OpenSSHKeyEncoder.exportOpenSSH() only checked for RSAPrivateCrtKey
• PEMEncoder.encodePrivateKey() only checked for RSAPrivateCrtKey
• Conscrypt's OpenSSLRSAPrivateKey implements RSAPrivateKey but not RSAPrivateCrtKey

Additionally, Ed25519 keys could not be exported in PEM format.

Solution

• Added handling for RSAPrivateKey (non-CRT) in both OpenSSHKeyEncoder and PEMEncoder
• Parse the PKCS#8 encoded form to extract CRT parameters and convert to RSAPrivateCrtKey
• Added encodeEd25519PrivateKey() to PEMEncoder using PKCS#8 format

Changes

File	Description
OpenSSHKeyEncoder.java	Handle non-CRT RSA keys via convertToRSAPrivateCrtKey()
PEMEncoder.java	Handle non-CRT RSA keys + add Ed25519 PKCS#8 encoding
OpenSSHKeyEncoderTest.java	Add tests for non-CRT RSA key export
PEMEncoderTest.java	Add tests for non-CRT RSA and Ed25519 encoding

Testing

• Added eight new tests covering non-CRT RSA and Ed25519 scenarios

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