Add VO methods to the base classes

Currently, we have 2 modules using pyvo, with at least one more in the works. So it's probably the right time to think about a common API and add VO related methods to the base class and/or template, e.g. `query_tap`, or `query_sia`, maybe even some properties. If the API would be sensible for the ESA modules, that rely on TapPlus, it would be even better (while I expect those modules could not really use the baseclass methods as is, but maybe this would help them to move to pyvo, too).

cc @keflavich @andamian @jespinosaar

Start by comparing the two existing pyvo-using modules with the ESA modules that rely on TapPlus; the issue does not name files or tests. Define the common VO API and determine whether it can support both groups, then document the agreed scope and validate it against the affected modules.

Written by the indexing model from the issue text.