feat: Implement `FromStr` for `AuthMechanism`

[duesee]

Oh, we can't as we want... FromStr can't return anything with a reference to the input string.

impl FromStr for AuthMechanism<'static> {
    type Err = ValidationError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        AuthMechanism::try_from(s.to_string())
    }
}

Close enough? See v2 branch.