Expose Codec.typeName

I'd like to propose that goavro exposes the typeName *name struct through a method on the codec. I feel that this would be beneficial to the users of goavro who might be interested in the fully qualified record name, have already built the codec, and would like to save cycles parsing the schema again for it.

E.g. Use of the fqn for schema registration in the Confluent Schema Registry where the Kafka naming strategy would be against the Record Name.

Start by locating the Codec type and the internal typeName name struct mentioned in the issue, then inspect existing Codec methods and their tests. Expose the fully qualified record name through the codec so callers can use it without reparsing the schema, and verify the returned name for a record schema.

Written by the indexing model from the issue text.