feat: Use `secrecy`

[soywod]

Would it make sense to use Secret from https://crates.io/crates/secrecy instead of the homemade imap_types::secret::Secret? It seems to bring a better level of security. I can propose a PR if interested in.

[duesee]

Yes, it does! I used it in the past but ditched it for some reason... Can't remember why honestly.

[soywod]

Interesting, I would be curious to know the reasons as well. I can still propose a PR on of these days and see.