[WIP] Json decoder factory v2

[scovich]

Which issue does this PR close?

This is a variant of #9259, but stacked on top of three building block PR:

• Move extension type construction logic out of Field #9266
• make_decoder accepts borrowed DataType instead of owned #9270
• Factor out json reader's static make_decoder args to a struct #9271

• Part of [arrow-json] deserialize Variant fields #8987

Rationale for this change

See description of #9259. This version here factors out building blocks so it's easier to see what actually changes to add custom json decoder support.

What changes are included in this PR?

See description of #9259. Same net change, just organized differently.

Are these changes tested?

Yes. Existing and newly added unit tests.

Are there any user-facing changes?

• Make JSON tape decoder classes public
• Make JSON ArrayDecoder trait public
• Make JSON DecoderContext class public
• New public trait: DecoderFactory

[scovich]

Oops @alamb this should prob stay in draft status until (a) the dependencies it's stacked on merge; and (b) we decide we actually want this approach in this form?