[Variant] make `serde_json` an optional dependency of `parquet-variant`

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

• @carpecodeum added support for writing variants as JSON as part of Variant: Write Variant Values as JSON #7670

However, #7670 also introduced a dependency on serde_json. As the various arrow and parquet crates are often low in the dependency stack and widely used, we are trying to avoid unnecessary dependencies

serde_json is a very helpful library for programmatically manipulating json objects, but many arrow usecases that use variant will not use this library (as the whole point of variant is to be more efficient)

Describe the solution you'd like

1. I would like the parquet-varaint crate to NOT depend on serde_json by default.
2. I would like parquet-variant to be a optional dependency -- so if people want easy / efficient serde_json conversion they can enable it

So like

# by default, does not have serde_json support
cargo test -p parquet-variant
# if the `serde_json` feature is added, now the serde_json code is available
cargo test -p parquet-variant --features=serde_json
# or also with --all-features
cargo test -p parquet-variant --all-features

Describe alternatives you've considered

Additional context

[Weijun-H]

take

[alamb]

FYI I think we should probably wait for this PR to merge:

• [VARIANT] Add support for the json_to_variant API #7783

I think then we'll be able to make a parquet-variant-json crate rather than making the dependency optional