add groups to user info output

[schiessle]

Example call:

curl -X GET http://admin:admin@localhost/master/ocs/v1.php/cloud/users/admin -H  "OCS-APIRequest: true"

Result:

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>100</statuscode>
  <message>OK</message>
  <totalitems></totalitems>
  <itemsperpage></itemsperpage>
 </meta>
 <data>
  <enabled>true</enabled>
  <quota>
   <free>338696790016</free>
   <used>7438874</used>
   <total>338704228890</total>
   <relative>0</relative>
   <quota>-3</quota>
  </quota>
  <email>[email protected]</email>
  <displayname>admin</displayname>
  <phone></phone>
  <address></address>
  <webpage></webpage>
  <twitter>schiessle</twitter>
  <groups>
    <element>group0</element>
    <element>group1</element>
  </groups>
 </data>
</ocs>

for nextcloud/android#591

[mention-bot]

@schiessle, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rullzer, @LukasReschke and @tomneedham to be potential reviewers.

[rullzer]

Fine by me 👍

[mario]

@schiessle so is this in 11.0.2?

[MariusBluem]

I see this commit not backported to stable11 ... so I think no? 🤔 @mario

[mario]

Thanks @MariusBluem. Can we at least get it in 11.0.3 @schiessle? :)