Only parse to the depth requested

There are cases in aggregation where an expand parameter might get
forwarded to a client.  Because our previous expand algorithm assumed
that any endpoint within bmcweb would only produce "depth=1" responses,
it was reasonable to assume that the pre-response could not contain
expanded content.  Aggregated resources can't make that assumption.

This commit attempts to pass through depth through the request, to
ensure that we only expand the level that the user requested, and not
any level returned by the request.  This is done by using the existence
of the resource identifer "@odata.id" to indicate each level in an
expanded response.  This should be fine since the Redfish spec requires
that property to exist.

Added unit tests to cover aggregation scenarios.  Modified existing
$expand tests to comply with the resource identifier dependency.

Tested:
New unit tests pass

Queried '/redfish/v1/Systems?$expand=.($levels=2)' on an aggregated
system whose satellite BMC supported $expand.  The overall response was
correctly expanded for both resources on the aggregating BMC as well as
on the satellite BMC.  Expanding the satellite resources did not require
sending multiple queries to the satellite.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I20ba60ee39bac11ffb3fe1768cec6299cf9ee13e
Signed-off-by: Carson Labrado <clabrado@google.com>

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