ENH: Add access for Australian astronomical data

I'm opening this issue here, so users and contributors of astroquery who may not follow the wider astropy project roadmap can see about the plans of adding AAO data access.
(I got a question recently about SkyMapper, and while encouraged them to open an issue and discussion, it highlighted that we haven't made any reference issue about the plans here in astroquery).

See more info: [AAO Research Data & Software](https://github.com/astropy/astropy-project/issues/426)

cc @NuriaLorente

Start by reading the linked astropy-project issue 426 and its AAO Research Data & Software plans. The astroquery files, tests, entry point, scope, and completion criteria are not identified in this issue, so those need to be established before implementation can begin.

Written by the indexing model from the issue text.