AVRO-2471: Generate specific conversions from LogicalType

[RyanSkraba]

Instead of collecting the types of primitive datum that appear in a generated class, use the model to find the appropriate Conversion<?> instances that should be used for logical types.

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[RyanSkraba]

There's some dead code as a consequence of this change: getClassNamesOfPrimitiveFields -- I'm taking a look.

I'm also thinking that if the user explicitly called addCustomConversion, it should be considered "used" and included no matter what, but that currently isn't the case.

[iemejia]

LGTM Thanks for fixing this Ryan using the registered logical types seems like the most reasonable approach.