Add runtime_mappings to search request (backport of #64374)

[nik9000]

This adds a way to specify the runtime_mappings on a search request
which are always "runtime" fields. It looks like:

curl -XDELETE -uelastic:password -HContent-Type:application/json localhost:9200/test
curl -XPOST -uelastic:password -HContent-Type:application/json 'localhost:9200/test/_bulk?pretty&refresh' -d'
{"index": {}}
{"animal": "cat", "sound": "meow"}
{"index": {}}
{"animal": "dog", "sound": "woof"}
{"index": {}}
{"animal": "snake", "sound": "hisssssssssssssssss"}
'

curl -XPOST -uelastic:password -HContent-Type:application/json localhost:9200/test/_search?pretty -d'
{
  "runtime_mappings": {
    "animal.upper": {
      "type": "keyword",
      "script": "for (String s : doc[\"animal.keyword\"]) {emit(s.toUpperCase())}"
    }
  },
  "query": {
    "match": {
      "animal.upper": "DOG"
    }
  }
}'

NOTE:
If we have to send a search request with runtime mappings to a node that
doesn't support runtime mappings at all then we'll fail the search
request entirely. The alternative would be to not send those runtime
mappings and let the node fail the search request with an "unknown field"
error. I believe this is would be hard to surprising because you defined
the field in the search request.

NOTE:
It isn't obvious but you can also use runtime_mappings to override fields
inside objects by naming the runtime fields with . in them. Like this:

curl -XDELETE -uelastic:password -HContent-Type:application/json localhost:9200/test
curl -uelastic:password -XPOST -HContent-Type:application/json localhost:9200/test/_bulk?refresh -d'
{"index":{}}
{"name": {"first": "Andrew", "last": "Wiggin"}}
{"index":{}}
{"name": {"first": "Julian", "last": "Delphiki", "suffix": "II"}}
'

curl -uelastic:password -XPOST -HContent-Type:application/json localhost:9200/test/_search?pretty -d'{
  "runtime_mappings": {
    "name.first": {
      "type": "keyword",
      "script": "if (\"Wiggin\".equals(doc[\"name.last.keyword\"].value)) {emit(\"Ender\");} else if (\"Delphiki\".equals(doc[\"name.last.keyword\"].value)) {emit(\"Bean\");}"
    }
  },
  "query": {
    "match": {
      "name.first": "Bean"
    }
  }
}'

Relates to #59332