Add support for fetching catalog queryables

[Samweli]

Implement API for getting queryables properties as defined in https://github.com/radiantearth/stac-api-spec/blob/master/fragments/filter/README.md#queryables, for the STAC catalogs that support additional data driven queriables.

[jsignell]

I'm a little torn on what the return type should be for this. I think the user is mostly interested in:

1. What are the fields in properties?
2. What are valid values for those fields?

The response type from the API is a jsonschema.

I guess in an ideal world there would be a new class that has at least the method: validate that can be used to validate any filter that is passed to ItemSearch.

But as a first implementation maybe a Dict[str, Any] is ok?