feat(core): add decimal type

[benjamin-awd]

Summary

This PR adds support for a new decimal type backed by the rust_decimal crate, providing 96-bit precision decimal arithmetic to solve long-standing floating-point precision loss issues (#1503, #755). Sibling Vector PR: vectordotdev/vector#25048.

Added a d'...' literal syntax for decimal values in VRL programs (e.g., d'0.12379999458789825'), consistent with VRL's existing quoted-literal conventions (r'...' for regex). Decimal literals are parsed at compile time into rust_decimal::Decimal values with no f64 intermediate.

parse_json(parse_float: "decimal") enables precision-preserving JSON ingress. Instead of deserializing through f64, this path uses serde_json::RawValue to capture each number's original string representation, then parses directly into Decimal. This avoids the two-step precision loss that causes issues like 0.12379999458789825 → 0.12379999458789824.

This is fully opt-in -- existing behavior is unchanged.

Note: moved away from the arbitrary_precision syntax since that's quite specific to serde_json and an implementation detail that some users may not understand, instead drew inspiration from Python's parse_float(decimal) syntax.

Change Type

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

Is this a breaking change?

• Yes
• No

We'll keep this opt-in. Within the VRL crate, unless the arbitrary_precision argument is passed in parse_json the default behavior will occur.

On the Vector side, it'll similarly be opt-in e.g.

sources:
  my_json_source:
    type: file
    include: ["/var/log/*.json"]
    decoding:
      codec: json
      json:
        parse_float: decimal  # Opt-in to parsing numeric data as decimal

How did you test this PR?

Tests within this PR and a sibling PR in Vector

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• 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.

Note: rust-decimal is already included in the LICENSE-3rdparty.csv file.

References

Closes #1692