feat(parse json): add arbitrary_precision for serde_json

[YJDoc2]

Summary

This PR adds a non-default feature called arbitrary_precision in this crate, which enabls the same in the serde_json dependency. Along with that, it also adds the correct(?) handling for serde_json's internal in-band representation of certain numbers. Also see this specifically the header TLS Shenanigans for more info on this that I referred.

As the feature is non-default, by default it should not cause any penalty to users who do not enable it. For those who do, I have tried to make it minimally impacting.

I am opening this PR first to get initial review on if I have done this in the correct place, any discussion about motivation of the change, and if this change is something of wont-fix category etc.

Notes to reviewers -

1. I will add corresponding in-code tests and any code comments after initial review
2. The reason I think the issue with floating numbers is caused (see test pr section) is because vrl "wraps" the serde_json type in its own KeyString and Value structs, so serde_json cannot perform its own checks for in-band values like normal. Hence we also need to add the same checks in this crate
3. I'd have preferred using the string from serde_json crate itself see here but the const is limited to crate visibility in serde_json, so cannot be used in this crate.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

For a minimal reproduction and fix check of this issue, use the following program :

1. Cargo.toml

[package]
name = "temp"
version = "0.1.0"
edition = "2024"

[dependencies]
serde_json = {version = "1.0.149", features = ["arbitrary_precision"]}
vrl = {path="..."} # for main branch
# vrl = {path="..." , features = ["arbitrary_precision"]} # for this pr

4. src/main.rs

use std::collections::BTreeMap;
use vrl::{
    compiler::{Context, TargetValue, TimeZone, state::RuntimeState},
    value,
    value::{Secrets, Value},
};

fn main() {
    let src = ".body,_ = parse_json(.body)\n.";
    let fns = vrl::stdlib::all();
    let result = vrl::compiler::compile(src, &fns).unwrap();
    let mut target = TargetValue {
        value: value!({body: "{\"v1\":\"abc\",\"v2\":123456.123}"}),
        metadata: Value::Object(BTreeMap::new()),
        secrets: Secrets::default(),
    };
    let mut state = RuntimeState::default();
    let timezone = TimeZone::default();
    let mut ctx = Context::new(&mut target, &mut state, &timezone);
    let value = result.program.resolve(&mut ctx).unwrap();
    let v = serde_json::to_value(&value).unwrap();
    println!("{:?}",v.to_string());
}

Note here that body itself is a json string, and v2is a number. Now if we run it without the arbitrary precision feature (i.e. main branch) , we get output as

"{\"body\":{\"v1\":\"abc\",\"v2\":{\"$serde_json::private::Number\":\"123456.123\"}}}"

If we run with the feature enabled, we get the output as

"{\"body\":{\"v1\":\"abc\",\"v2\":123456.123}}"

I believe second is the correct one. Also this is specific to floats, if we make the number just 123456 , it works as expected on main branch as well.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines. : Will do it after first review.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here. : N/A
• For new VRL functions, please also create a sibling PR in Vector to document the new function. : N/A

References

• Closes: Provide arbitrary_precision option for serde_json float conversions #1503
• Closes: parse_json - differs parsing floats #755 (?)