Add bt_ctf_field_structure_set_field() function

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add CTF Writer type serialization stubs

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change the event fields implementation to reuse the structure management code

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