feat: automatic idempotency tokens for header values

[kuhe]

https://smithy.io/2.0/spec/behavior-traits.html#idempotencytoken-trait

Client implementations MAY automatically provide a value for a request token member if and only if the member is not explicitly provided.

This adds the automatic default token for headers. Previously added for query params in aws/aws-sdk-js-v3#4272.

before

const headers: any = map({}, isSerializableHeaderValue, {
  "content-type": "application/json",
  [_xact]: input[_CT]!,
});

after

import { v4 as generateIdempotencyToken } from "uuid";

const headers: any = map({}, isSerializableHeaderValue, {
  "content-type": "application/json",
  [_xact]: input[_CT] ?? generateIdempotencyToken(),
});