Add Kafka Connect as a built‑in JMX metrics target

[aaaugustine29]

Overview:
This change introduces Kafka Connect as a first‑class JMX target system in the JMX metrics library. It adds a ruleset and documentation that cover both Apache Kafka Connect and Confluent Platform variants from the outset, so users can enable Kafka Connect monitoring without custom YAML.

Details:
Added kafka-connect.yaml JMX rules that map worker, rebalance, connector, task, source/sink task, and task-error MBeans into OpenTelemetry metrics, including Apache‑only metrics (e.g., worker rebalance protocol, per‑connector task counts, predicate/transform metadata, converter metadata, source transaction sizes, sink record lag max).
Defined connector and task status as state metrics using the superset of status values across Apache and Confluent, to avoid vendor‑specific enum mismatches.
Documented the new target in kafka-connect.md, including metric groups, attributes, and the dual‑vendor compatibility model (no renames; Apache list as a superset of Confluent docs).
Added self‑contained tests for the Kafka Connect rules that load the YAML, build metric definitions, and validate key state mappings and metric presence, ensuring the new target is ready to consume from day one.

Testing:
./gradlew -Dorg.gradle.configuration-cache.parallel=false instrumentation:jmx-metrics:library:test

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: aaaugustine29 / name: Aaron Augustine (d20153d, 2f25096, 31b44c8, 6293d68, 81fa539, c985e86, d46eb77, dd17633)