Simplified API for writing to JSON

[jorgecarleitao]

This PR privatizes the JsonFormat trait and replaces it by a simple enum.

The trait is an implementation detail of this crate - the relevant aspect is that the user can choose between JSON and NDJSON, which can be achieved by a simple enum.

Backward incompatible changes

To migrate

• replace arrow2::io::json::write::JsonArray::default() by arrow2::io::json::write::Format::Json
• replace arrow2::io::json::write::LineDelimited::default() by arrow2::io::json::write::NewlineDelimitedJson

[codecov]

Codecov Report

Merging #864 (4b2dff6) into main (10e6cd5) will increase coverage by 0.00%.
The diff coverage is 91.66%.

@@           Coverage Diff           @@
##             main     #864   +/-   ##
=======================================
  Coverage   71.56%   71.57%           
=======================================
  Files         335      335           
  Lines       17952    17961    +9     
=======================================
+ Hits        12847    12855    +8     
- Misses       5105     5106    +1     

Impacted Files	Coverage Δ
src/io/json/write/serialize.rs	86.11% <83.33%> (-0.22%)	⬇️
src/io/json/write/mod.rs	100.00% <100.00%> (ø)

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