astroquery.eso should make good use of http://archive.eso.org/wdb/wdb/eso/gravity/prog

After discussions with the support team of the ESO archive, I understand that there is a /prog endpoint for each query /from. This endpoint gives a very convenient list of the available options.
This is more or less a note to myself to make good use of this function for option discovery and parameter checks.

Start by reading the astroquery.eso query/from implementation and inspecting the ESO archive's /prog endpoint for its available options. Clarify which queries should use it and what parameter checks are required; done means option discovery and validation are integrated with appropriate tests.

Written by the indexing model from the issue text.