Add CI Job token support to Maven packages API

What does this MR do?

Add CI Job token support to Maven packages API. You can build and upload maven packages from GitLab CI job

ci-settings.xml

<settings xmlns="http://maven.apache.org/SETTINGS/1.1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.1.0 http://maven.apache.org/xsd/settings-1.1.0.xsd">
  <servers>
     <server>
       <id>gitlab-maven</id>
       <configuration>
         <httpHeaders>
           <property>
             <name>Job-Token</name>
             <value>CI_JOB_TOKEN</value>
           </property>
         </httpHeaders>
       </configuration>
     </server>
   </servers>
 </settings>

.gitlab-ci.yml

deploy:
  script:
    - 'cp ci-settings.xml /root/.m2/settings.xml'
    - 'sed -i "s/CI_JOB_TOKEN/${CI_JOB_TOKEN}/g" /root/.m2/settings.xml'
    - 'mvn deploy'
  only:
    - master
  image: maven:3.3.9-jdk-8

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5811

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?