Add stream_event_context accessors

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: Only allow setting a packet context on a non-frozen stream class

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Cleanup: Missing whitespace in comment

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tests: Add unit tests for stream event context accessors
Cleanup: Remove unnecessary argument name

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add stream_event_context accessors

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tests: Add unit tests for stream event context accessors
Cleanup: Misplaced end label in bt_ctf_stream_get_packet_context

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Docs: Add a comment about event validation in bt_ctf_stream_append_event

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>