Add helper to return &mut SslRef from stream

[jmayclin]

Currently there is no way to retrieve a mutable reference to the underlying SslRef given some SslStream. This PR adds an ssl_mut method to do that.

[jmayclin]

There is a failure in the CI, but it looks to be unrelated to my PR.

error: package `cc v1.0.95` cannot be built because it requires rustc 1.63 or newer, 
while the currently active rustc version is 1.56.0

cc bumped their MSRV in the latest release (3 days ago): https://github.com/rust-lang/cc-rs/releases/tag/1.0.95, which appears to be the reason for this failure.

[botovq]

CI should be fixed once you rebase on main