createSecretKey and hmac (e.g. in fastify/cookie)

I wanted to have an open discussion regarding the use of secret keys in hmac.

I think we can have performance improvements by using createSecretKey. Atleast it seems that the heap usage could be reduced.

We should consider investigating this?!

I am opening this issue to not forget about it. It is 2 o clock in the morning and I saw this potential perf improvement while checking the performance of a different product. Good night.

@kibertoad
@Fdawgs
@Eomm
@gurgunday
@mcollina