* Scripting: fill in get contexts REST API

Updates response for `GET /_script_context`, returning a `contexts`
object with a list of context description objects.  The description
includes the context name and a list of methods available.  The
methods list has the signature for the `execute` mathod and any
getters. eg.
```
{
  "contexts": [
     {
       "name" : "moving-function",
       "methods" : [
         {
           "name" : "execute",
           "return_type" : "double",
           "params" : [
             {
               "type" : "java.util.Map",
               "name" : "params"
             },
             {
               "type" : "double[]",
               "name" : "values"
             }
           ]
         }
       ]
     },
     {
       "name" : "number_sort",
       "methods" : [
         {
           "name" : "execute",
           "return_type" : "double",
           "params" : [ ]
         },
         {
           "name" : "getDoc",
           "return_type" : "java.util.Map",
           "params" : [ ]
         },
         {
           "name" : "getParams",
           "return_type" : "java.util.Map",
           "params" : [ ]
         },
         {
           "name" : "get_score",
           "return_type" : "double",
           "params" : [ ]
         }
       ]
     },
...
  ]
}
```

fixes: elastic#47411