AVRO-2639: add option to write Optional only when fields are nullable

[ecopoesis]

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Jira

• My PR addresses the following issues and references them in the PR title. For example, "AVRO-1234: My Avro PR"
  • AVRO-2639
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• My PR adds the following unit tests OR does not need testing for this extremely good reason:

• testPojoWithOptionalOnlyWhenNullableCreatedTurnedOn

• testPojoWithOptionalOnlyWhenNullableCreatedTurnedOnAndGettersReturnOptionalTurnedOff

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[kan-izh]

Nice! While you are here, I think it is a good idea to tweak it further: Use OptionalInt/OptionalLong/OptionalDouble for corresponding primitive types.

[Bluexin]

This looks like it implements the same functionality as #817, but that implementation's velocity template seemed a bit cleaner at first glance.
Regardless, is there something blocking this or the other PR? I don't mind helping out.

[daicode-jan]

Would be nice to have an update on this important feature

[Fokko]

Thanks for the PR, I like this one. Would be much nicer if we could use the primitive types, as @kan-izh suggests.