im: use pjsip api to create and parse messages

This supports both simple and multipart content types.

*WARNING* the daemon API has not changed; however the previous
implementation was not RFC compliant and thus messages sent from or
received by a previous version of the daemon will not be interpreted
correctly. Additionally the behaviour of the API has slightly changed.
It will no longer split multiple message pairs received in the
map<string, string> into multiple, separate SIP messages if they do not
all fit into one. If there is only one pair in the map, then a message
with a single content-type will be created and sent. If multiple pairs
exist, then a multipart/mixed type message will be created. If the
created message is too large to be sent, then no parts will be sent.

Support for storing the URI of the original sender is dropped for now as it
was not being used by the clients and the implementation was buggy.

The APIs for creating XML recipient lists as defined by RFC 5365 remains
but is no longer used for now.

Issue: #79657
Change-Id: I2b00cbd797fbb423ee0a7eb24748d2362e9f9ff8
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>