MM-24037 Adding getKnowUsers API endpoint (#14332)

* Adding getKnowUsers API endpoint

* Adding i18n strings

* Fixing golint errors

* Adding doc strings

* Remove debug line

* Updating app_iface

* Fixing gofmt

i18n/en.json

@@ -7242,6 +7242,10 @@
     "id": "store.sql_user.get_for_login.multiple_users",
     "translation": "We found multiple users matching your credentials and were unable to log you in. Please contact an administrator."
   },
+  {
+    "id": "store.sql_user.get_known_users.get_users.app_error",
+    "translation": "Unable to get know users from the database."
+  },
   {
     "id": "store.sql_user.get_new_users.app_error",
     "translation": "We encountered an error while finding the new users."