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>