Implements metadata query for complex query feature

A subfield in the resource_metadata field of the Sample
can be used in filter expression as
  metadata.<subfield_name>

Validation of the names of the metadata is not implemented

Implements: blueprint complex-filter-expressions-in-api-queries
Change-Id: Icf198f740191b5fff0008e9aa7b054c1cc33d75d
Merge "Implements metadata query for complex query feature"