Support multiple timestamps in featuresearch/featureidentify search

To fully support the new time compare (layers_timestamp) feature of the map application, we need to extend the existing featuresearch and featureidentify search to support per-layer time settings. Right now, only one timeInstant can be specified.

This PR adds an additional timeStamps parameter, which allows to support the requirement. This PR is backwards compatible (it behaves exactly the same as before for existing parameters).

See also discusion here: https://github.com/geoadmin/mf-geoadmin3/pull/1784