feat: allow http(s).Agent ctor arg in lieu of instance

[kuhe]

Simplify usage by allowing http(s).Agent constructor argument instead of an instance of such.

Before:

import https from "node:https";
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { NodeHttpHandler } from "@smithy/node-http-handler";

const client = new DynamoDBClient({
  requestHandler: new NodeHttpHandler({
    requestTimeout: 3_000,
    httpsAgent: new https.Agent({
      maxSockets: 25
    }),
  }),
});

After:

import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { NodeHttpHandler } from "@smithy/node-http-handler";

const client = new DynamoDBClient({
  requestHandler: new NodeHttpHandler({
    requestTimeout: 3_000,
    httpsAgent: { maxSockets: 25 },
  }),
});

End goal:

import { DynamoDBClient } from "@aws-sdk/client-dynamodb";

const client = new DynamoDBClient({
  requestHandler: {
    requestTimeout: 3_000,
    httpsAgent: { maxSockets: 25 },
  },
});

// the longer forms will still be valid