Gaia timeouts affecting imports

Hi,

This line is causing issues: https://github.com/astropy/astroquery/blob/main/astroquery/gaia/core.py#L1190. What happens is that in the same file where `GaiaClass` is defined, it's instantiated as `Gaia`. However when the Gaia servers are down / timing out, this means that just importing code from that file can take > 30 seconds. This can be a big issue as relying on astroquery.gaia as a dependency can affect your codebase as a whole, slowing things down a lot.

It would be best if these classes were not instantiated in the same file as they are defined, to avoid breaking/slowing imports.

Start in astroquery/gaia/core.py at line 1190 and inspect how GaiaClass and Gaia are imported and used. Separate class definition from instantiation so importing the Gaia module does not wait on unavailable or timing-out Gaia servers. Done means importing astroquery.gaia remains prompt when the service is down.

Written by the indexing model from the issue text.