Add async Kotlin client support to the Java SDK

[shaealh]

Closes #67683

Why

The Java SDK currently exposes blocking task and client APIs. Kotlin tasks need coroutine-native equivalents so suspending functions can call Airflow APIs without blocking for each request.

What

• Add coroutine-native AsyncClient and AsyncTask APIs.
• Add an asynchronous coordinator transport and task-runner dispatch.
• Preserve the existing synchronous Client and Task behavior.
• Extend BuilderProcessor to recognize Kotlin suspend functions.
• Support Context, XCom inputs, and non-Unit return values in generated asynchronous tasks.
• Add KAPT integration, suspension, concurrency, failure, and client-forwarding tests.
• Document Kotlin coroutine task usage.

Testing

• ./gradlew clean build --console=plain -Pkotlin.incremental=false
• ./gradlew :sdk:check :processor:check --console=plain -Pkotlin.incremental=false
• Repository commit hooks passed, except lychee-docker, which could not run because Docker was unavailable.

Was generative AI tooling used to co-author this PR?

[X] Yes (OpenAI Codex)