feat: Add support for runtime_mapping

[jmaitrehenry]

Fix #152

Add the support for runtime_mapping like:

const field = esb.runtimeField(
      'keyword',
      `emit(doc['sessionId'].value + '::' + doc['name'].value)`,
);
const reqBody = esb.requestBodySearch();
reqBody.runtimeMapping('sessionId-eventName', field);

This will generate a query like that:

{
  "runtime_mappings": {
    "sessionId-eventName": {
      "type": "keyword",
      "script": {
        "source": "emit(doc['sessionId'].value + '::' + doc['name'].value)"
      }
    }
  }
}

[sudo-suhas]

will try to check this during the weekend. Thanks @jmaitrehenry for the PR.

[jmaitrehenry]

Thanks for the review @sudo-suhas, I will try to update it in the next few days.

[sudo-suhas]

ping @jmaitrehenry

[pimpelsang]

please finish it :)

[atreids]

Hi, I've raised a separate pull request based on @jmaitrehenry 's work to try and get this feature merged. @sudo-suhas If you have some time to review, that would be greatly appreciated.