SONAR-6867: Add a new APIs UserDetails.getUserId()/setUserId(String userId)

SONAR-6867: Add a new APIs UserDetails.getUserId()/setUserId(String userId)
to allow plugins to pass userId information to SonarQube

setUserId(String userId) and getUserId () are added to UserDetails.

Testing
 1. Unit test for UserDetails
 2. Verification of following scenarios :
    a. Plugin (e.g. LDAP) is dependent on older sonarqube plugins apis
       ( < 5.2) and the plugin is being used in SonarQube 5.2 server
    b. Plugin is dependent on SonarQube 5.2 plugin apis and userId is not
       being populated in UserDetails along with userName and Email .
    c. Plugin is dependent on SonarQube 5.2 plugin apis and userId is
       also being populated by the plugin in UserDetails along with
       userName and Email.