Create pipeline job templates covering gradle lifecycle

Gradle has a somewhat different lifecycle compared to Maven. Instead of standardized phases to execute, it appears custom tasks need to be specified. A first pass at this would be to create a generic gradle job expecting a TASK argument, like so:

.gradle:
 variables:
  TASK: ""
 script:
  - gradle $TASK

which could then be utilized like so:

test-gradle-job:
 extends: .gradle
 stage: test
 variables:
  TASK: test