Add optional type

Add optional type

The `SIDE_TYPE_OPTIONAL' denotes a field that can be absent from the
trace.  Such field has no attribute.  Instead, the optional type is
simply a pointer to another type.

The determination of absence for an optional field is determined by the
`selector' member of the `side_arg_optional' structure, which must be
the constant value `SIDE_OPTIONAL_ENABLED' if the field is present,
otherwise `SIDE_OPTION_DISABLED'.

Change-Id: Ie23bcac27efef66938981d59b715a950465ede75
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>