Fix: Implements generic aggregations for multisearch

[hoiekim]

This PR fixes the following bug (#1937)

import { Client } from "@elastic/elasticsearch";
import { AggregationsTermsAggregateBase } from "@elastic/elasticsearch/lib/api/types";

const client = new Client({ node: "http://localhost:9200" });

const reproduce = async () => {
  interface Doc {
    foo: string;
  }

  interface Aggregations {
    unique: AggregationsTermsAggregateBase<{ key: string }>;
  }

  const response = await client.msearch<Doc, Aggregations>({
    index: "foo",
    searches: [
      {},
      {
        query: { match_all: {} },
        aggregations: { unique: { terms: { field: "foo" } } }
      }
    ]
  });

  response.responses.map((e) => {
    if ("error" in r) return
    return e.aggregations?.unique.buckets;
    //                            ^^^^^^^
    // Property 'buckets' does not exist on type 'AggregationsAggregate'.
    // Property 'buckets' does not exist on type 'AggregationsCardinalityAggregate'.
  });
};

The error will go way with this commit

Similar PR: #1596

[JoshMock]

And there's the PR I saw coming. 🙂 I'll take a look soon.

[github-actions]

This pull request is stale because it has been open 90 days with no activity. Remove the stale label, or leave a comment, or this will be closed in 14 days.

[JoshMock]

Closing. See: #1937 (comment)