Client policy for TLS connections. This issue will provide the ability for
clients to specify a callback (listener) function that gets invoked for
certificate verification and authentication of outbound TLS connections from the
JAIN-SIP stack. In order to provide both authentication and encryption, a TLS
client must verify that the certificates presented in the TLS handshake are
valid by verifying that the certificate is signed by a trusted CA. This issue
will define and implement listener for this purpose.