Merge Spring Integration Kafka into Main Repo

- merge master branch from old repo (`git-filter-repo`) for src branch
- update build.gradle
- copy docs from Spring for Apache Kafka Chapter

Doc polishing.

Fix http: references in tests.

* Add `What's New` note and fix `polling-consumer.adoc`
not linking to external project any more

build.gradle

@@ -98,6 +98,7 @@ ext {
 	soapVersion = '1.4.0'
 	springAmqpVersion = project.hasProperty('springAmqpVersion') ? project.springAmqpVersion : '2.3.0-SNAPSHOT'
 	springDataVersion = project.hasProperty('springDataVersion') ? project.springDataVersion : '2020.0.0-SNAPSHOT'
+	springKafkaVersion = '2.5.3.BUILD-SNAPSHOT'
 	springSecurityVersion = project.hasProperty('springSecurityVersion') ? project.springSecurityVersion : '5.4.0-SNAPSHOT'
 	springRetryVersion = '1.3.0'
 	springVersion = project.hasProperty('springVersion') ? project.springVersion : '5.3.0-SNAPSHOT'
@@ -592,6 +593,24 @@ project('spring-integration-jpa') {
 	}
 }
 
+project('spring-integration-kafka') {
+	description = 'Spring Integration for Apache Kafka'
+	dependencies {
+		api  project(':spring-integration-core')
+		api ("org.springframework.kafka:spring-kafka:$springKafkaVersion") {
+			exclude group: 'org.springframework'
+		}
+
+		testImplementation "org.springframework.kafka:spring-kafka-test:$springKafkaVersion"
+		testImplementation "com.willowtreeapps.assertk:assertk-jvm:$assertkVersion"
+		testImplementation 'org.jetbrains.kotlin:kotlin-reflect'
+		testImplementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
+
+		testRuntimeOnly 'com.fasterxml.jackson.core:jackson-core'
+		testRuntimeOnly 'com.fasterxml.jackson.core:jackson-databind'
+	}
+}
+
 project('spring-integration-mail') {
 	description = 'Spring Integration Mail Support'
 	dependencies {