feat: 🎸 added custom properties

[jtmarks6]

Summary

Added custom properties like Apache Avro SchemaBuilder has: https://avro.apache.org/docs/1.7.6/api/java/org/apache/avro/SchemaBuilder.html. This will allow schema to contain custom metadata on fields which will greatly increase the use cases of Avro schemas.

Description

All fields now have the method .prop(string, any) which will add a custom property to the field. This is the same syntax and output schema as Apache's SchemaBuilder:

Testing

Added unit tests for all types.

Issue

This PR will resolve issue #1

[jtmarks6]

Please consider if the custom property should be duplicated here on top and nested level. I thought since the name is duplicated, the custom prop also should be. This also applies to a few other types. (Any that have the nested/duplicate name)