pjsip: New function PJSIP_PARSE_URI to parse URI and return part of URI

New dialplan function PJSIP_PARSE_URI added to parse an URI and return
a specified part of the URI.

This is useful when need to get part of the URI instead of cutting it
using a CUT function.

For example to get 'user' part of Remote URI
${PJSIP_PARSE_URI(${CHANNEL(pjsip,remote_uri)},user)}

ASTERISK-28144 #close

Change-Id: I5d828fb87f6803b6c1152bb7b44835f027bb9d5a