Add explicit implementation dep. for reactor

We noticed IntelliJ was giving strange errors in CPM2.0 when not having
the webflux or reactor dependency. The cause of this was most probably
that the reactor dependency was not transitively available through
webflux.

This commit adds the reactor dependency explicitely in the hope that it
is transferred correctly to projects using Labradoor as a dependency.

build.gradle.kts

@@ -242,8 +242,12 @@ dependencies {
     // Dependency for mapping one data class to another
     implementation("org.modelmapper", "modelmapper", "2.3.6")
 
+    // Collections dependency for dependency scanning
     implementation("commons-collections", "commons-collections", "3.2.2")
 
+    // Explicit reactor for use in those who use this library
+    implementation("io.projectreactor", "reactor-core", "3.2.11.RELEASE")
+
     // Test dependencies
     testImplementation("org.springframework.boot", "spring-boot-starter-test", springBootVersion) {
         exclude("junit", "junit")