feat(codecs): make avro codec optional via cargo feature flag

[ahachete]

Summary

Make the avro codec optional via a new codecs-avro Cargo feature flag, mirroring the existing gating pattern already used in lib/codecs/Cargo.toml for arrow, parquet, opentelemetry, and syslog codecs.

The feature is enabled by default in every release feature aggregator (base, and transitively default-no-api-client, the per-target release features, etc.). Both sources-pulsar and sinks-pulsar pull codecs-avro automatically. No behavior change for anyone using the default build, the shipping release binaries, or the existing release feature sets.

Custom builds via cargo build --no-default-features --features ... that do not enable codecs-avro (and do not enable sources-pulsar or sinks-pulsar) skip compiling apache-avro and the avro codec implementation. Measured savings: ~440 KB on the production release profile (opt-level=3, lto=fat, codegen-units=1, panic=abort, strip=symbols) for a curated build.

This is a small, self-contained step toward the broader goal in #20064. Quoting @jszwedko in that umbrella issue:

the real savings is likely to come from users only compiling in the modules they need ... we do enable that via feature flags, but it's not well documented or easy for users to create their own distributions

Vector configuration

No configuration change. Existing YAML using codec: avro continues to work because codecs-avro is enabled by default in release builds.

How did you test this PR?

Compile-time:

• cargo check --no-default-features --features "sources-file,transforms-remap,sinks-console,sinks-opentelemetry": apache-avro no longer linked.
• cargo check --no-default-features --features "sources-file,transforms-remap,sinks-console,sinks-pulsar": codecs-avro auto-pulled via sinks-pulsar, Pulsar's SerializerConfig::Avro match arm in src/sinks/pulsar/config.rs:370 compiles unchanged.

Runtime:

• Production release build (same minimal feature set, profile: opt-level=3, lto=fat, codegen-units=1, panic=abort, strip=symbols) shrinks from 30.600 MB to 30.166 MB (−444 KB).

Change Type

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

(Closest fit. The change is purely structural: it adds a build-time opt-out without altering any runtime behavior).

Is this a breaking change?

• Yes
• No

The feature is enabled by default in every release feature aggregator and pulled in transitively by both Pulsar features, so default and shipping builds are bit-equivalent in behavior.

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.

Changelog fragment included: changelog.d/codecs_avro_optional.enhancement.md.

References

• Related: Reduced size binary #20064 (umbrella issue: Reduced size binary)
• Related: fix(releasing): enable codecs-parquet in all release feature sets #25321 (precedent: codecs-parquet enabled in release feature sets). Same release-aggregator update pattern is followed here for codecs-avro.

Notes for reviewers

• The new feature follows the existing pattern in lib/codecs/Cargo.toml (and the *Serializer / *Deserializer enum gating in serializer.rs / decoding/mod.rs) for arrow, parquet, opentelemetry, syslog. Each new #[cfg(feature = "avro")] site mirrors a matching site for one of those codecs.
• src/components/validation/resources/mod.rs already had #[cfg(feature = "codecs-opentelemetry")] and #[cfg(feature = "codecs-syslog")] gates on the corresponding match arms; this PR adds the analogous #[cfg(feature = "codecs-avro")] gates in two places.
• [[bin]] generate-avro-fixtures now declares required-features = ["avro"], the standard cargo idiom for feature-gated binaries.
• The root Cargo.toml keeps the existing apache-avro workspace dep at v0.16 unchanged — that is used directly by the Pulsar source/sink for protocol-level types and is independent of the codec dep at v0.20.0 in lib/codecs/Cargo.toml.

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