Add an ASDF schema for the TETE coordinate frame

[chris-simpson]

Description

Not all coordinate frames in astropy.coordinates.builtin_frames have ASDF serialization schemas. This means that it may be impossible to write data with properly-defined gWCS objects to disk, either as pure ASDF files or, as DRAGONS (Gemini DR) does, as FITS files with ASDF-like extensions. One of those missing is astropy.coordinates.TETE and it would be helpful to have a schema for this.

Additional context

I don't claim to understand all the nuances of coordinate schemes so perhaps there's a reason why some coordinate frames have schemas and some don't, or perhaps it's simply limited resources to add each new frame? But I've got some data with RADESYS="GAPPT" and I'd like to make an ASDF-serializable gWCS object from it. There's an archived discussion on GAPPT here but @StuartLittlefair informs me that TETE is the appropriate reference frame to use.

[pllim]

I moved this from astropy.