Aggregation: Process subordinate top collections

Adds a function to process responses from URIs that are uptree from
a top level collection.  A follow-up patch will hook this into the
aggregation code to allow adding links to top level collections which
are only supported by satellite BMCs.

Adds test cases to validate this function is working correctly.

Tested:
New test cases pass

Signed-off-by: Carson Labrado <clabrado@google.com>
Change-Id: I7f0fd6c3955398e2fde136c1d3b37a6bf4bf06b9