Draft: adapt Vector to VRL ObjectMap

[lukesteensen]

Summary

Adapts Vector to the experimental VRL ObjectMap work by replacing direct
BTreeMap assumptions with ObjectMap-compatible construction and iteration
patterns.

VRL dependency used for this draft:

• Branch: https://github.com/lukesteensen/vrl/tree/experiment/objectmap-backends
• Pinned commit: d243198bc134ab19ea6293322d36055fd54c2fe2

Commit structure

The main ObjectMap work is isolated in:

• refactor: adapt Vector to VRL ObjectMap
• chore: use git VRL ObjectMap dependency

This branch also includes two small test fixes that were discovered while
testing the ObjectMap changes locally, but are otherwise unrelated to ObjectMap:

• test(codecs): avoid mutating global log schema in syslog tests

  • Avoids syslog tests mutating global log_schema state, which can pollute
    other tests depending on execution order.

• test(vector-core): keep empty DDSketch fixtures canonical

  • Avoids generating non-canonical empty DDSketch fixtures, which can fail
    round-trip expectations.

Benchmark support is included separately in:

• chore(bench): add ObjectMap benchmark helpers

Validation

Passed:

• cargo fmt --all -- --check
• cargo check --workspace --all-targets --all-features --locked
• make check-clippy
• cargo test -p vector-core --all-features --lib --locked
• cargo test -p codecs --all-features --lib --locked
• targeted Vector tests around map conversion, HTTP headers, and sink encoding
• targeted VRL object_map tests

Benchmark sanity check:

• Case: datadog_agent_remap_blackhole
• Current btree -> current flat: +24.43%
• Short local run: --replicates 1 --total-samples 60 --warmup-seconds 15