Add `ProxyType` support to `AvroConverter.from_recap`

[criccomini]

AvroConverter does not currently handle ProxyTypes. This is somewhat nuanced since the converter must support both aliases that are defined in the RecapType being passed in as well as Recap's standard aliases (int32, int64, etc).

I think the the implementation should just do {"type": alias} if the ProxyType's alias has already been defined in the Avro schema. If not, it the implementation should call .reslove() on the ProxyType and use the resolved type (with the alias set as the Avro name, so it's defined for future use).