LexiconUnknownUnion fails to account for fully unknown/arbitrary values

[luciascarlet]

The ATProto documentation states that an unknown type in a lexicon has no type-specific fields and can be anything:

unknown#

Indicates than any data could appear at this location, with no specific validation. Note that the data must still be valid under the data model: it can't contain unsupported things like floats.

No type-specific fields.

This library uses a LexiconUnknownUnion union distinguishing between a set of known structs and relying on a $type field as a coding key. This does not, however, work if there is no $type field present in the response at all, as with didDoc in the output of createSession for example. Attempting to decode such a response causes a DecodingError.

It appears that this is incorrect behaviour, as no type validation is stated to be needed for an unknown type at all. These should be decoded differently in a manner that can represent any arbitrary JSON objects, for example using JSONSerialization to output an array. Unfortunately that produces a dynamic Any type, so I really am not sure how you might go about integrating this into a system built around Codable, which is very strict about types.