Swap `ring` for our own in-crate RSA validator

Swap `ring` for our own in-crate RSA validator

While `ring` is great, it struggles with platform support and has a
fairly involved dependency tree due to its reliance on C backends.

Further, while the `RustCrypto` org tries to stick to Rust, in
doing so it takes on more (unnecessary) dependencies and has a
particularly unusable MSRV policy. Finally, its contributor base
has historically not been particularly friendly.

Thus, sadly, there's not really a good option for doing RSA
validation using a third-party crate.

Instead, we go our own way here, adding an in-crate RSA validator.