Support for service redirects

Your opinion please.

This is a request from ALMA but it might apply to other services too. Their top site `almascience.org/tap` automatically redirects to one of the mirror sites. They would like to see the direct searches to the top URL work which currently is not possible because `POST` redirects are no supported.

The way we solved this problem in `astroquery.alma` was to do a `GET` on the capabilities of the top URL and use the resulted URL for the subsequent searches/actions.

Do people think it would be a good idea to add something similar to the service constructors in `PyVO`? Is it worth the extra overhead of the `GET` call for this enhancement? I'll do the implementation if the feedback is positive.

Start with the PyVO service constructors and the existing GET capabilities flow described in the issue. Determine whether constructors should resolve redirected service URLs before subsequent searches and actions, and clarify the expected behavior and cost of the extra GET request before implementation.

Written by the indexing model from the issue text.