[Webadmin] corresponding routes for JMX LISTUSERDOMAINMAPPINGS command

you will implement the *UserMappingRoutes* which does

GET /mappings/user/{username}
eg.
GET /mappings/user/blob@james.org

Return 200 status code with a json body represent for a list of *MappingValueDTO*

[
 {
   "type": "alias",
   "mapping": "destination12@address1.domain"
 },
 {
   "type": "group",
   "mapping": "destination12@address2.domain"
 }
]

• Returns 400 status code with invalid parameter values
• Returns 500 status code for server error