ctf: Expose map of attributes in Packet Descriptor

ctf: Expose map of attributes in Packet Descriptor

The motivation is to allow accessing custom fields in CTF
packet contexts. It decouples the CTF parser from LTTng a bit.

This breaks an unused api.

Change-Id: I2bead35a7e080d4e3ab325395629c44eea455cfc
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/61815
Reviewed-by: Hudson CI
Reviewed-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>