lib: add BLOB field class types

lib: add BLOB field class types

Add support for BLOB field classes and fields, with the intent of
supporting CTF2‑SPEC‑2.0's BLOB field class types [1].

A BLOB is a sequence of binary data included as-is in the trace.

A BLOB field class has a media type property, which describes the kind
of content its instances hold.

There are three variations of a BLOB field class, analogous to the array
field classes:

 - static length BLOB
 - dynamic length BLOB without a length field
 - dynamic length BLOB with a length field

Static length BLOBs have a fixed length, set in the field class.

Dynamic length BLOBs have a variable length, set in each field instance
of the field class.

Dynamic length BLOBs with a length field have a length field location,
set in the field class, pointing to a preceding field that holds the
length of the BLOB.

Philippe updated the documentation.

[1] https://diamon.org/ctf/CTF2-SPEC-2.0.html#blob-fc

Change-Id: Ib9d7bd12d598fbc6b7ed5d80d8cdfcf294e4254d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7350
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12687