Implement FromStr for SecretString

[azhur]

This could be convenient for use cases like using secrecy + https://github.com/greyblake/envconfig-rs that rely on FromStr

[tony-iqlusion]

Since the conversion is infallible, I've added a From<&str> impl instead: #1237

[azhur]

@tony-iqlusion We are using envconfig crate and it requires FromStr to be implemented so it won't work having just From<&str>. The previous secrecy versions had FromStr implemented, now we can't upgrade secrecy so I'm wondering If we can add back FromStr implementation.

I'm happy to create a PR if you agree to add FromStr.

[tony-iqlusion]

Ok, we can add a FromStr impl with Err = Infallible

[cemoktra]

when do you plan to publish this? i'm also running into this during update