KEY-31 - Add support for EC Keys and ECDSA Signing operations

Adds ability to load elliptic curve keys alongside RSA keys in private key directory. EC public keys are identified by SHA256 digest of OpenSSL's representation as a hexadecimal string (from `EC_POINT_point2hex`).
Adds support for new KSSL opcodes for ECDSA signing, corresponding to the RSA opcodes masked with `0x10`.
Cleanup of testclient, allowing testing with public keys (RSA and EC) instead of private keys.