sessiond: field: add field roles and blob types

Extend the field/field type to include the blob field types that
are used to express the trace class uuid in CTF2.

Roles defined by the CTF2 specification are also added to allow a trace
class to define the layout of some scopes (e.g. packet header and
context) instead of hard-coding the serialized version of the layout
description.

The TSDL trace class visitor takes the new types into account by
converting them into arrays of "bytes" (uint8_t).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ic9d976416ed765b157e951bf9b8b299abbe20e5b