Single .kt file output without nested classes for schema generator

[jangalinski]

When we generate (data) classes from schema, we still use a RootDataClass strategy with nested sub classes for embedded types.
This is not a good idea as the FQN of the inner class does not macht the FQN of the schema (see #138).

TODO: AS we do with protocol already, rewrite the generator for AvroSchemaDeclaration to create a single kotlin source file that contains all types mentioned in the schema, but without nesting.

This is a breaking change, so we rename the milstone to 1.12.4.0