Scripting: fill in get contexts REST API (#48319)

[stu-elastic]

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: #47411

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[stu-elastic]

@elasticmachine run elasticsearch-ci/packaging-sample-matrix

[jdconrad]

LGTM! Please add the version you're going to backport to as a label. 7.5, right? Also, were you able to test this locally against Java 8? I did my best to look through this carefully, but a compiler is definitely better than my eyes for detecting something that might not be J8 compatible.

Edit: NM, CI passed which should test the compatility.