secrecy: Add an example to deserialize a SecretString

[Vrixyz]

I had to do a bit of searching to allow a SecretString to be serialized.

Here is my solution:

fn expose_secret_string<S>(data: &Secret<String>, ser: S) -> Result<S::Ok, S::Error>
where
    S: serde::Serializer,
{
    ser.serialize_str(data.expose_secret())
}
#[derive(Debug, Serialize, Clone)]
pub struct SecretInside {
    #[serde(serialize_with = "expose_secret_string")]
    pub secret: Secret<String>,
}

I understand this is dangerous implementation, but as long as it's carefully explained, and still opt-in with such boilerplate, I suggest to add this snippet to the documentation around here:

crates/secrecy/src/lib.rs

Line 253 in d86b82b

/// [`serialize_with`][2] attribute to specify a serializer that exposes the secret.

Let me know if you're interested in a PR 😄