feat(honeycomb sink): add samplerate_field config option

[fru1tworld]

Summary

Add a samplerate_field configuration option to the Honeycomb sink. When set, the specified field is extracted from each event and sent as the samplerate value in the Honeycomb Batch API. The field is removed from the event data. If the field is missing or not an integer, samplerate is omitted.

Closes #18098

Vector configuration

[sinks.honeycomb]
type = "honeycomb"
api_key = "${HONEYCOMB_API_KEY}"
dataset = "my-dataset"
samplerate_field = "sample_rate"

How did you test this PR?

• Added 6 unit tests covering: valid integer, no config, missing field, non-integer type, zero value, and mixed batch
• cargo test -p vector --lib -- sinks::honeycomb::tests::samplerate (6/6 pass)
• make check-clippy pass
• make check-fmt pass

Change Type

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

Is this a breaking change?

• Yes
• No

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.

References

• Closes: Support the samplerate property in the Honeycomb sink #18098

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[fru1tworld]

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[fru1tworld]

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