ssh-key: added optional support for verifying legacy ssh-rsa SHA1 signatures

[Eugeny]

FYI @tarcieri

[Eugeny]

Heya don't want to push this too hard but merging this and having a .pre crate release would allow me to publish a beta of the new ssh-key and ssh-encoding powered https://github.com/warp-tech/russh (high level SSH client/server crate) release ✌️

[tarcieri]

@Eugeny that sounds great, unfortunately I'm on hiatus and cutting releases is a bit non-trivial due to the dependencies, but I will try to take a look in a week or so

[baloo]

Any chance you could add a test here?

[Eugeny]

@baloo done

[tarcieri]

Heya don't want to push this too hard but merging this and having a .pre crate release

@Eugeny hello from many, many months later. Sorry this has been dragging on for so long, but there is a release out now: https://crates.io/crates/ssh-key/0.7.0-rc.0

[Eugeny]

No worries, glad the things are picking up pace again