Provide a Catalog API server

[forman]

Is your feature request related to a problem? Please describe.

The API served by xcube serve should be extended by a catalog API. This is urgently required to let users query (interactively) what datasets and variables are provided, and how they are named so they can be referenced, e.g.

• to open and access the actual data;
• to configure the input sections in processing requests for the xcube gen2 tool.

Individual data stores can already describe themselves and the contained datasets (#307 and #369), but this is not yet possible for a pool of data stores (#364).

Describe the solution you'd like

• Compatibility of queries with some standard, e.g. OpenSearch Geo and Time Extensions
• Compatibility of results with some metadata standard, e.g. STAC Collection.
• Implemented as a plugable extension to existing xcube serve tool, i.e. we need to specify server API extensions so plugins can contribute.
• Allow xube serve to use configured data store pools the same way it is done for the xcube gen --stores <store-pool.yaml> ....