TMF: Add support for StructDefinition fields to ctfadaptor

TMF: Add support for StructDefinition fields to ctfadaptor

CTF supports recursive field definitions. So does TMF now.
* Added the CTFStructField class
* Updated the unit tests for this new class

Change-Id: I5e476da2a73f172ff946a1fff386e6fb48dff1bc
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/10401
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>