GH-3195 Mark new Kotlin runtime dependencies as optional

[seonwooj0810]

Fixes #3195

Summary

kotlin-reflect and kotlinx-coroutines-reactor were added to core/spring-cloud-stream/pom.xml (commit 11d5fb3 "Adjusted Kotlin dependencies") with <scope>runtime</scope> but without <optional>true</optional>. As a result, Java-only Spring Cloud Stream consumers transitively pull these Kotlin artifacts into their fat jars.

This mirrors the situation that #3128 addressed for kotlin-stdlib-jdk8, which sits a few lines above in the same POM and is already marked optional. The fix is to add <optional>true</optional> to both new dependencies, matching that established precedent.

Change

 		<dependency>
 				<groupId>org.jetbrains.kotlin</groupId>
 				<artifactId>kotlin-reflect</artifactId>
 				<scope>runtime</scope>
+				<optional>true</optional>
 			</dependency>
 			<dependency>
 				<groupId>org.jetbrains.kotlinx</groupId>
 				<artifactId>kotlinx-coroutines-reactor</artifactId>
 				<scope>runtime</scope>
+				<optional>true</optional>
 			</dependency>

The two dependencies remain runtime-scoped and are still available to applications that opt in (Kotlin consumers add them explicitly, or pick them up via spring-boot-starter-webflux or similar). Consumers that do not use Kotlin are no longer forced to ship them.

Test evidence

POM-only change — no source files touched.

• ./mvnw -N validate on core/spring-cloud-stream — BUILD SUCCESS.
• ./mvnw -N help:effective-pom -Doutput=/tmp/eff-pom.xml on the same module; resolved dependencies on the project:

  org.jetbrains.kotlin:kotlin-stdlib-jdk8       scope=compile optional=true   (pre-existing)
  org.jetbrains.kotlin:kotlin-reflect           scope=runtime optional=true   (this PR)
  org.jetbrains.kotlinx:kotlinx-coroutines-reactor scope=runtime optional=true   (this PR)
  

Skipped the full multi-module build because the change is pure POM metadata (the optional flag does not affect compilation or test behavior of this module — only how the dependency is propagated transitively to downstream consumers).

Verification done: (1) gh pr list for "kotlin-reflect optional", "3195 kotlin" — no in-flight PR; (2) issue #3195 has zero comments, no self-claim; (3) confirmed fix touches POM only and follows the established <optional>true</optional> pattern set by #3128 in the same file; (4) ran mvn help:effective-pom to confirm optional=true resolves on both dependencies in the project view; (5) reviewed git history — commit 11d5fb3 introduced the new deps without the optional flag, and no follow-up commit fixed it.