AVRO-3989: [java] add conversion classes to default instance of SpecificData

[hmit]

What is the purpose of the change

This pull request improves deserialization of data when using default SpecificData instance.

Why?
Often times, SpecificDatumReader and ReflectDatumReader are configured using the default SpecificData instance. This leads to deserialization error when reading fields with union type and inner logical type (see test-case).

Deserialization works correctly when using the custom SpecificData instance stored in the MODEL$ field of the schema class that extends SpecificRecord. The custom SpecificData has the necessary conversions stored to handle fields with union and logical types. The issue has been reported multiple times, notably in [AVRO-3989](linked JIRA).

Verifying this change

This change added tests and can be verified as follows:

• The added test passes when the code is present but fails when the code is removed

Documentation

• Does this pull request introduce a new feature? (no)

[hmit]

CC: @Fokko I'd appreciate it if you could look at this PR and leave any feedback for me! Also, apologies for the 1:1 ping in advance.