feat(codecs): add support for decimal type

[benjamin-awd]

Summary

This PR adds support for a new decimal type backed by the rust_decimal crate. Sibling VRL PR: vectordotdev/vrl#1719 (issue: vectordotdev/vrl#1692).

Note: this requires the VRL PR to be merged first.

This feature should be considered in beta, some additional work will have to be done in the future to wire up existing sinks to work with the decimal type.

Benchmark results:

Single-event results

Payload	Baseline	Decimal Precision	Overhead
simple	~975 ns	~962 ns	~0% (noise)
numeric	~901 ns	~1,007 ns	~12%
high_precision	~716 ns	~782 ns	~9%
mixed	~1,396 ns	~1,651 ns	~18%

Batch results

Batch Size	Baseline (thrpt)	Decimal (thrpt)	Overhead
100	92.8 MiB/s	77.4 MiB/s	~17%
1,000	92.7 MiB/s	84.2 MiB/s	~9%
10,000	98.9 MiB/s	83.6 MiB/s	~15%
100,000	101.2 MiB/s	84.1 MiB/s	~17%

Vector configuration

How did you test this PR?

Tests in VRL and Vector

Change Type

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

Is this a breaking change?

• Yes
• No

This shouldn't have any breaking changes unless decimal parsing is enabled.

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.