func_pjsip_endpoint: Add PJSIP_ENDPOINT function for querying endpoint details

This patch adds a new function, PJSIP_ENDPOINT, which lets the dialplan query,
for any endpoint, any property configured on an endpoint. This function is a
companion to the CHANNEL function, which can be used to extract the endpoint
name for a channel.

Review: https://reviewboard.asterisk.org/r/3035


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@403616 f38db490-d61c-443f-a65b-d21fe96a405b
func_channel, chan_pjsip: Add CHANNEL read function support for chan_pjsip

This patch adds CHANNEL read support for chan_pjsip. This allows the dialplan
to use the CHANNEL function on a chan_pjsip channel to obtain run-time
information about the channel from the PJSIP channel driver and the PJSIP
stack. This includes:
 * RTP information, including source/destination media addresses, whether or
   not the media is secure, held, and other properties.
 * RTCP information. This includes sets of parseable information, as well as
   individual statistic attriutes.
 * PJSIP information. This includes URIs, local/remote signalling addresses,
   whether or not the signalling is secure, and other properties.
 * The endpoint name. This can be used in conjunction with the PJSIP_ENDPOINT
   function to obtain more detailed endpoint information.

Review: https://reviewboard.asterisk.org/r/3038/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@403618 f38db490-d61c-443f-a65b-d21fe96a405b