Add /mappings endpoint

Lists AbstractUrlMappings and AbstractHandlerMethodMappings
in a loose Map structure. E.g.

{
  "/**/favicon.ico": {
    "bean": "faviconHandlerMapping"
  },
  "/**": {
    "bean": "resourceHandlerMapping"
  },
  "/webjars/**": {
    "bean": "resourceHandlerMapping"
  },
  "{[/error],methods=[],params=[],headers=[],consumes=[],produces=[],custom=[]}":
     {
       "bean": "requestMappingHandlerMapping",
       "method": "public java.util.Map<java.lang.String, java.lang.Object> org.springframework.boot.actuate.web.BasicErrorController.error(javax.servlet.http.HttpServletRequest)"
...
}

Fixes gh-378