#23 Add support for key and trust stores with certificate alias

[Daniel Morris]

Would like to request that SIPp add support for key stores and trust stores (p12 and JKS) on the client and allow for certificate alias selection from the key store rather than using PEM files. This allows the private keys to be stored in a password protected key store and would allow for granting trust to multiple server entities.

Suggest adding options along the following lines:
-tls_certificate_alias - certificate alias to use on connection
-tls_key_store - password protected keystore containing
-tls_key_store_password - password for the keystore
-tls_trust_store - trust store containing trusted certs
-tls_trust_store_password - password for the trust store