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.

[jturkel]

It looks like the spec supports metadata attributes on all JSON object in the serialized Avro schema. Is there a generalized version of this change that supports attributes on constructs other than just fields e.g. records?

[hughevans]

@jturkel Yes, I can make that change no problem. Would you rather seperate config blocks for declaring the attributes for for each object type? Or is all together ok?

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

[jturkel]

I'm not familiar with how people are using metadata on Avro schemas but my instinct is to keep it simple for now with a single set of attributes that applies to all types/fields. We can always extend this in the future to support both global and type specific metadata attributes.

[hughevans]

@jturkel Fantastic. I’ve pushed that change. There is the possibility still to have individual setters for records vs fields in the future if you want too.

[jturkel]

It looks like the Avro spec supports metadata on all types rather than just record types. If that's correct, then should this be on Avro::Builder::Types::Type rather than Avro::Builder::Types::RecordType?

[hughevans]

@jturkel sorry I haven’t got back to this because I hit some code here that could benefit from a refactor. Any possibility we could merge this as is and then circle back later?