Extend Encryption UDFs component with support for encrypted private keys

When asymmetric private keys (RSA, DSA or EC) are generated / used, the user should be allowed to specify a password to encrypt their content.

Here is how UDF extensions may look like:

• create_asymmetric_priv_key(@algorithm, @key_len | @dh_parameters | @curve_name [, @password_enc_algorithm, @password])
• create_asymmetric_pub_key(@algorithm, @priv_key [, @password_enc_algorithm, @password])
• asymmetric_sign(@algorithm, @digest, @priv_key, @digest_type [, @password_enc_algorithm, @password])
• asymmetric_verify(@algorithm, @digest, @signature, @pub_key, @digest_type [, @password_enc_algorithm, @password])
• asymmetric_encrypt(@algorithm, @str, @key [, @password_enc_algorithm, @password])
• asymmetric_decrypt(@algorithm, @crypt_str, @key [, @password_enc_algorithm, @password])