feat: add requestHandler ctor pass through type

[kuhe]

Currently, to override any options in the default request handler, the requestHandler class must be instantiated.

This adds a type representing default request handler ctor params,
intended to allow the following syntactic sugar:

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

var agent = new https.Agent({
  maxSockets: 25
});

var dynamodbClient = new DynamoDBClient({
  requestHandler: new NodeHttpHandler({
    httpsAgent: agent
 })
});

the PR's ultimate goal is to allow the following:

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

var agent = new https.Agent({
  maxSockets: 25
});

var dynamodbClient = new DynamoDBClient({
  requestHandler: {
    httpsAgent: agent
  }
});

This may appear to be a very mild simplification, but it avoids user errors like accidentally installing a locked version of @smithy/node-http-handler or needing to differentiate between the previous @aws-sdk/node-http-handler and its migrated name.

[syall]

should these types and interfaces live in @smithy/types, and the @smithy/*-http-handler packages import them so there is no duplication?

[kuhe]

yes, let's do that