swift: add Codable implementation for sum-of-product types

[tadfisher]

Support sum-of-product types in Swift by generating Codable implementations.

There are a couple of dimensions here, so I attempted to cover them all in tests:

• encoding style: TaggedObjectStyle, TaggedFlatObjectStyle
• case constructor: no fields, single unnamed field (linked record type), multiple named fields (only seen in TaggedFlatObjectStyle with record constructors)

In order to support "unknown" cases instead of throwing an error, the decoding logic needs to know the "unknown" field name at codegen time, so we are strongly nudged into making "unknown enum cases" a supported feature. This is done by adding enumUnknownCase :: Maybe String to Options and a corresponding enumEnumUnknownCase to MoatEnum. Only sum-of-product decoding takes this into account when decoding; the unknown case is generated for all enums as it doesn't hurt anything, but clients will have to implement their own Codable extensions to fall back to the unknown case for bare ("C-style") enums. We could implement that if desired, but I left that for a future improvement.

[tadfisher]

Swift models diff here: https://github.com/MercuryTechnologies/mercury-web-backend/pull/25210/files#diff-fc7abc0a0d261322ca6274b8c1aa926e0d46c23a93a67ba5946cd5c9b8df94e8

[josephsumabat]

LGTM from the Haskell end. My swift knowledge is limited but based on our internal implementation this all seems reasonable to me!