Support big-decimal logical type (avro 1.12.0)

[Chuckame]

Avro 1.12.0 introduced a big-decimal which is a decimal without having the scale and precision information in the schema, allowing more flexibility, and so removing the need of AvroDecimal.

Currently, a BigDecimal without the AvroDecimal annotation indicating scale and precision fails.

Now, it would use by default the big-decimal logical type, while the decimal type is kept when the annotation is present.