Allow specify dynamic templates in bulk request

[dnhatn]

Backport of #69948 to 7.x

This change allows users to specify dynamic templates in a bulk request.

PUT myindex
{
  "mappings": {
    "dynamic_templates": [{
      "time_histograms": {
        "mapping": {
          "type": "histogram",
          "meta": {
            "unit": "s"
          }
        }
      }
    }]
  }
}

// Tells ES to use the dynamic template named `time_histograms` for field `response_times`
// in the index request
POST myindex/_bulk
{ "index": { "dynamic_templates": { "response_times": "time_histograms" } } }
{ "@timestamp": "2020-08-12", "response_times": { "values": [1, 10], "counts": [5, 1] }}

Closes #61939