feat(parsers.avro): Support multiple modes for union handling

[athornton]

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resolves #12970

Added a config item to allow user to specify "flatten" (current behavior), "nullable" (what the issue requestor wanted, and what we at Rubin Observatory need internally), and "any" (just take what comes in and put it in the named field) as modes by which Avro can handle unions.

The common use case of "nullable"--a field that can sometimes be null/missing, and if so, we get a measurement without that metric--is the major enhancement.

Either "nullable" with multiple non-null types or "any" would make InfluxDB unhappy with the received data (the entire measurement will be dropped when InfluxDB sees a metric of a type other than whatever it first saw (for a given shard)), but if getting data into InfluxDB isn't your goal, those modes could be useful.

[athornton]

Once #13939 is merged I'll rebase.

[athornton]

I have a question that comes from @afausti -- what should the default behavior be if you explicitly mark an incoming field as both a tag and a field? The current code "does both" which ... I don't know what that does in practice. The right answer is "don't do that," of course...but should I catch it when building the parser and warn and leave it a tag but not add it as a field?

[srebhan]

Thanks for tackling this @athornton! Just two small comments...

[athornton]

Review suggestions applied, and rebased onto current master.

[srebhan]

Thanks for the update @athornton!