feat: configurable extra allowable metadata field attributes

[hughevans]

According to the Avro specification any attribute can be added to a field and used as extra metadata provided it does not clash with the other attributes specified.

To enable the use of specific custom metadata attributes you can define them with:

Avro::Builder.extra_metadata_attributes(
  :sensitivity_level,
  :deprecated_by,
  :reference
)

I also solved #9 when adding this.