This commit adds the `crypto.createSecretKey` API.
Key management: This follows the same approach as our WebCrypto
CryptoKey impl where we use WeakMap for storing key material and a
handle is passed around, such that (only internal) JS can access the key
material and we don't have to explicitly close a Rust resource.
As a result, `createHmac` now accepts a secret KeyObject.
Closes https://github.com/denoland/deno/issues/17844
