Query parameters are specified as key=value pairs, multiple pairs
must be delimited with ampersands (&), and only a single value should be
specified for a particular key. The following query parameter keys are
allowed:
creator
scope
identifier
revision
url
If a query parameter is specified, and a subscription's respective attribute
does not match it, that subscription will not be included in the group of
subscriptions returned. If scope, identifier, or revision are used, their
values must together constitute a syntactically and semantically correct EML
packageId (i.e. "scope.identifier.revision") - either partial or complete.
If url is used, its value must not contain ampersands. Therefore, if a
subscription's URL contains ampersands, it cannot be filtered based on its
URL.