feat: allow commands to be constructed without arg if all arg fields optional

[kuhe]

currently:

new GetCallerIdentity({}); // empty argument required
sts.getCallerIdentity({}); // empty argument required

new PutObjectCommand({ Key: "", Bucket: "" }); // non-empty argument required
s3.putObject({ Key: "", Bucket: "" }); // non-empty argument required

in this PR, feature:

new GetCallerIdentity(); // argument may be omitted
sts.getCallerIdentity(); // argument may be omitted (codegen TODO)

new PutObjectCommand({ Key: "", Bucket: "" }); // non-empty argument still required
s3.putObject({ Key: "", Bucket: "" }); // non-empty argument still required

[kuhe]

For aggregated clients, a no-arg signature will be added if no required members are found.

  /**
   * @see {@link ListBucketsCommand}
   */
  listBuckets(): Promise<ListBucketsCommandOutput>;
  listBuckets(args: ListBucketsCommandInput, options?: __HttpHandlerOptions): Promise<ListBucketsCommandOutput>;
  listBuckets(args: ListBucketsCommandInput, cb: (err: any, data?: ListBucketsCommandOutput) => void): void;
  listBuckets(
    args: ListBucketsCommandInput,
    options: __HttpHandlerOptions,
    cb: (err: any, data?: ListBucketsCommandOutput) => void
  ): void;

[kuhe]

this addresses aws/aws-sdk-js-v3#4579