Decimal logical type schema json representation is inconsistent with Java impl

[Ten0]

I initially wrote it correctly because that's how I understood it following the spec and was what felt like it had the most sense, but then when my interop tests with apache-avro were failing I assumed I got it wrong and spent 2 hours doing this refactor:
8f73223 (#9)

Turns out, of course it was the rust apache-avro crate that was wrong, and I should have kept the previous implementation 😑:
https://issues.apache.org/jira/browse/AVRO-3925

=> We should revert this. (logical type attributes should be flattened with the original type and name)
Unfortunately this probably means that we should also revert the schema representation update because that means that the original one was more appropriate (notably with regards to name resolution) if indeed we don't need to support the current representation, but that's a breaking change so it will require a new major version. ☹️