include moc in shape (polymorphism)

[pdowler]

The shape xtype allows for circle, range, and polygon.

It describes the POS query parameter of SIAv2 aka DAP.

shape or multi-shape (TBD) can more or less be used for the s_region column in ObsCore with minimal work/effect.

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Use cases that seem related to shape and moc:

1. Output: My service(s) and database use moc as the internal representation of the footprint of ObsCore observations and I want to be able to output the values as-is without conversion.
2. Input-Query: My service(s) use moc and I'd like to accept queries using moc (DAP and TAP)
3. Input-Content: TAP users upload a table with a moc column: what could/should/must the service do?