fix(enriching): csv file enrichment tables no longer drop the first row

[B-Schmidt]

Summary

• Currently, file enrichment tables loaded from a csv file without headers loose the first row of data in that file
• Fix this by keeping the first data row in a separate variable and then chaining it into the remaining data rows when parsing columns
• Provide unit tests for loading a csv file both with and without header row

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?

• Unit tests are provided in the PR
• Create data.csv

a,1
b,2

• Create vector.yaml

enrichment_tables:
  data:
    type: file
    file:
      encoding:
        type: csv
        include_headers: false
      path: ./data.csv

sources:
  in:
    type: stdin

transforms:
  remap:
    inputs: [in]
    type: remap
    source: |
      .row = get_enrichment_table_record("data", {"0": .message}) ?? null
      . = {"message": .message, "row": .row}

sinks:
  out:
    type: console
    inputs: [remap]
    encoding:
      codec: json

• Run vector -c vector.yaml providing the following input:

a
b
c

• Expected output:

{"message": "a", "row": {"0": "a", "1": "1"}}
{"message": "b", "row": {"0": "b", "1": "2"}}
{"message": "c", "row": null}

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

• Please read our Vector contributor resources.
  • make check-all is a good command to run locally. This check is
    defined here. Some of these
    checks might not be relevant to your PR. For Rust changes, at the very least you should run:
    • cargo fmt --all
    • cargo clippy --workspace --all-targets -- -D warnings
    • cargo nextest run --workspace (alternatively, you can run cargo test --all)
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run dd-rust-license-tool write to regenerate the license inventory and commit the changes (if any). More details here.

References

[pront]

Thanks @B-Schmidt!

A more idiomatic way would be to use reader.records().peekable() but I can see how that can get complicated with the existing implementation.