ENH: Irsa.query_ssa to handle format keyword better

There are some inconsistencies between the standard/server side docs/pyvo docs, so at the moment `'all'` is hardwired, but we should support other formats, too.

See discussion: https://github.com/astropy/astroquery/pull/3076#discussion_r1962567503

This is potentially an upstream issue both for IRSA and pyvo

Start with the query_ssa entry point and read the linked discussion in pull request 3076. Compare the standard, server-side, and pyvo format expectations, then determine which non-'all' formats should be supported and whether the change belongs upstream. Done means the format keyword behaves consistently with the agreed specification.

Written by the indexing model from the issue text.