Tune Gradle to make our builds much faster

* `org.gradle.parallel`: Enabling parallel builds makes the build
  considerably faster, that said, we need to make sure tests don't depend
  on each other, which should be already the case.

* `org.gradle.vfs.watch`: Since Gradle 6.5, avoids unnecessary I/O.

* `org.gradle.caching`: Shared caches can reduce the number of tasks we need to execute by
  reusing outputs already generated. This can significantly decrease build times but we
  need to make sure our tests do not depend on other external services,
  which is already the case.

gradle.properties

@@ -14,4 +14,9 @@
 # limitations under the License.
 #
 systemProp.nebula.features.coreBomSupport=true
-kotlin.code.style=official
+kotlin.code.style=official
+
+org.gradle.caching = true
+org.gradle.parallel = true
+# org.gradle.configureondemand = true
+org.gradle.vfs.watch = true