Improve `MastMissions` queries by enabling criteria decorators

The MastMissions API requires parameters to be fed as a JSON dictionary, which means the syntax restructuring that's performed by `services.py`'s `_build_catalogs_params` is not done for these queries, and therefore criteria decorators are not enabled for MastMissions the way they are for Catalogs (see documentation examples [here](https://astroquery.readthedocs.io/en/latest/mast/mast.html#mission-searches)) Recently the PANSTARRS API switched to requiring params to be passed in a JSON dictionary so I did some refactoring to allow for this while also preserving the criteria decorator capabilities (https://github.com/astropy/astroquery/pull/2727) . This can also be done with MastMissions queries.

Start in services.py at _build_catalogs_params and compare the MastMissions query path with the PANSTARRS refactoring in pull request 2727. Review the MastMissions documentation examples for mission searches. Done means MastMissions accepts JSON-dictionary parameters while criteria decorators remain available as they are for Catalogs.

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