Adding secrecy or zeroize for EncodingKey and DecodingKey

[mettke]

It probably makes sense to either add the https://crates.io/crates/secrecy or the https://crates.io/crates/zeroize crate to this crate. This way we can make sure that private and secret keys are securely erased from memory when dropped.

[Keats]

You mean in a custom Drop impl? I've used zeroize before so i would lean towards that

[arckoor]

If we want to be really paranoid it might make sense to zeroize PemEncodedKey as well
Everything passed to us as &(str|[u8]) the user should obviously clear themselves, but if we want to be clean with all cryptographic material the intermediates could be cleaned as well I guess
That said, #483 is much better than not having it at all, so I'll soon-ish go ahead with that
I think #499 (comment) still has merit if anyone wants to look at that (might require some additional consideration in regards to HSMs or similar - I'm tbh not even sure how the CryptoProvider already fits in with those)