feat: enable ctor arg passthrough for requestHandler

[kuhe]

This implements step 3 of the following progression mentioned earlier in #1165.

Simplifying provision of custom settings to the requestHandler.

Previously:

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
    }),
  }),
});

Step 2 (implemented):

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 },
  }),
});

Step 3 (this PR):

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

AWS SDK sample diff in aws/aws-sdk-js-v3#5820

[kuhe]

ClientDefaults is on the input side, and should not have been assigned the ResolvedConfig version of "SmithyConfiguration". This correction is backwards compatible.

[kuhe]

This is an input config corresponding to the resolved config below. So, it can be loosened.