Make logical types extensible

[AlexeyRaga]

{ "name": "Decimal", "type": {
      "type": "bytes", "logicalType": "decimal", "precision": 7, "scale": 2
}}

As per Avro specification, logical types are optional for codecs to implement.
This library implements "basic" logical types, but they are hard coded into the library.

It would be nice to be able to make logical types extensible so that users of the library would be able to provide their own logical types.

Currently, Java and even .NET implementations allow "registering" new logical types.
We ought to be able to do it, too ;)

[AlexeyRaga]

I wonder if it can be implemented somehow like

class LogicalType (name :: Symbol) baseType haskellType | name -> haskellType where
    fromBaseType :: Schema -> baseType -> Either String haskellType
   ...

and instances.

Questions: how to make TH generator see that and know to generate appropriate logical types?..