introduce new builtin crypto.parse_private_keys

* crypto.parse_private_keys parses private keys, returns a list of valid
keys
* consolidated getPrivateKeysFromString  with getPrivateKeysFromPEMData
for crypto.x509.parse_rsa_private_key  and crypto.parse_private_keys

* reworked crypto.x509.parse_rsa_private_key so that we no longer need
getPrivateKeysFromString  but instead determine type of key in builtin
function and added input validation checks

Co-authored-by: Charlie Egan <git@charlieegan3.com>
Signed-off-by: Emil Volckmar Ry <emilvry@gmail.com>