feat: Add secrecy crate feature

[scott-rc]

Hey 👋

This PR adds the ability to use secrecy::Secret<T> from the secrecy crate as an sqlx::Type.

Example:

#[derive(Debug, FromRow)]
pub struct User {
    pub id: i32,
    pub email_address: String,
    pub password_hash: secrecy::Secret<String>,
}

[abonander]

I'm personally not inclined to merge this because I still stand by what I stated in #613. Using secrecy is just plugging one hole in a very leaky dam and I'm worried about luring people into a false sense of security.

If you really care about security then using secrecy should be only one small part of a comprehensive hardening effort. Until such effort materializes I feel like using secrecy is just security theater.

The one thing I can get behind with secrecy is that it prevents logging of sensitive information, which is pretty important. If that was all it purported to do I'd include it in a heartbeat.

[scott-rc]

@abonander Your concerns are fair and I didn't notice #613 before making this PR.

I'll close this for now.