Add Kotlin version for micronaut-reactor-streaming-http-client #1751

The guide guides/micronaut-reactor-streaming-http-client is missing a Kotlin version.

Expected work:

• Add the Kotlin sample implementation for the guide.
• For multi-application guides, add the Kotlin implementation under each application directory that needs it.
• Update guides/micronaut-reactor-streaming-http-client/metadata.json to include KOTLIN in languages.
• Run ./gradlew micronautReactorStreamingHttpClientBuild --stacktrace.
• Run ./gradlew micronautReactorStreamingHttpClientRunTestScript --stacktrace.

This issue was created from an audit of guides missing Java, Groovy, or Kotlin variants. Groovy variants are intentionally not requested for Graal/GraalVM-specific guides.