Add Couchbase cache support

This commit updates the cache auto-configuration to provide a
`CouchbaseCacheManager` if a `Bucket` has been configured.

The global customizer infrastructure allows to further tune the cache
manager if necessary.

Closes gh-5176

spring-boot-autoconfigure/pom.xml

@@ -35,6 +35,11 @@
 			<artifactId>transactions-jta</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>com.couchbase.client</groupId>
+			<artifactId>couchbase-spring-cache</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>com.fasterxml.jackson.core</groupId>
 			<artifactId>jackson-databind</artifactId>