Consolidate JSON Reader options and DecoderOptions

[alamb]

Which issue does this PR close?

Draft as it builds on #1451 from @sum12 .

Closes #1538

Rationale for this change

@sum12 started refactoring options into a common struct in #1451 and I wanted to finish that work to minimize API churn when we release the next arrow

What changes are included in this PR?

1. Move batch_size to DecoderOptions as well.
2. Consolidate field handling in ReaderBuilder

Are there any user-facing changes?

Yes -- if they use the low level Json decoder, the options are now encoded in a struct rather than as parameters

[alamb]

this default is moved into DecoderOptions

[alamb]

I would like to merge this prior to creating the arrow 12 release candidate later this week (so that we minimize the API churn) -- @nevi-me and @houqp do you perhaps have a moment to review this?

cc @sum12

[tustvold]

It took me a while to work out why there is a separation between the Decoder and a Reader and therefore we end up with nested builders, but I think this makes sense. We want to allow people to construct a Decoder with arbitrary streams of serde_json::Value and convert them to RecordBatch, whereas Reader only supports files 👍

[alamb]

It took me a while to work out why there is a separation between the Decoder and a Reader and therefore we end up with nested builders, but I think this makes sense. We want to allow people to construct a Decoder with arbitrary streams of serde_json::Value and convert them to RecordBatch, whereas Reader only supports files 👍

I tried to encode this insight into the docs here: #1559