Unhide methods in astroquery.gaia

E.g. currently we have `__query_object` that is called in both `query_object` and `query_object_async`, but kept inaccessible to look at from a python prompt. I see no reason for having all these three methods defined while the latter two only calls on the implementation in the first.

Instead, we should rename the hidden to `query_object_async`, and use the decorator to create the `query_object`, just as we do in the other modules.

Start in the astroquery.gaia module by locating __query_object, query_object, and query_object_async. Compare their structure with the corresponding methods in other modules, then verify that the implementation is renamed and the decorator exposes the synchronous method without retaining redundant wrappers. Done means the public methods remain functional and the hidden implementation is no longer needed.

Written by the indexing model from the issue text.