[WIP] Expose unread messages count in API's contacts list

WIP because it seems that the unread counter is never set to 0.

Fixes #1746
1 job for 1746-headless-unread-counter in 9 minutes and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #6066
test

00:09:31

 
Name Stage Failure
failed
test Test

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BUILD FAILED in 7m 2s
139 actionable tasks: 118 executed, 21 up-to-date
Running after script...
$ rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
$ rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/
ERROR: Job failed: exit code 1