wcst_import - objectExist endpoint should return if a coverage / layer exist as local or remote object

[Bang Pham Huu]

From this doc ​https://doc.rasdaman.org/05_geo-services-guide.html?highlight=objectexists#check-if-a-coverage-layer-exists

rasdaman/ows/objectExists only returns true/false without saying the coverage is local or remote.

In this case, wcst_import sees a remote coverage as a local coverage and tries to update it which is not allowed (exception in petascope).

If coverage/layer exists, it should return new format true:local and true:remote for local coverage and remote coverage accordingly.