Implement Key Pair Generator

[paullatzelsperger]

Implement a KeyPairGenerator, that can create KeyPair objects from generator parameters specified in a JSON structure (not JWK!). Depending on which keys are to be generated, this JSON structure must hold the appropriate values.

As a first step, we will support: RSA, EC and Ed25519 (EdDSA) keys:

{
   "algorithm": "EC | RSA | EdDSA",
   "properties": [
       "length": 2048 
       "curve": "OIDname or stdname for EC and EdDSA"
   ]
}

• The curve field must be the std name, not the OIDs or any other aliases, e.g. "secp256r1", not "1.2.840.10045.3.1.7" or "NIST-P256" for NIST-P256. Must be one of ["Ed25519", "X25519"] for EdDSA keys (no other curves supported).
• if algorithm is "EC", but no curve is specified, then a "secp256r1" is generated.
• The length field is only relevant for RSA keys and defaults to 2048.
• if the algorithm field is null or empty, all properties are ignored and an Ed25519 key is generated.

NB: this class will initially be created in IH, but may get upstreamed later