Support gating libssh-rs and ssh2 behind features of same name

[chipsenkbeil]

Moves libssh-rs and ssh2 backends to be gated by features of the same name. This enables building wezterm-ssh with only one of two dependencies, getting around problems with compiling both for a cdylib target.

You can verify that each backend works by running with cargo test --no-default-features --features libssh-rs and cargo test --no-default-features --features ssh2 from within the wezterm-ssh directory. If libssh-rs is not present, then the backend will be switched to ssh2.

[chipsenkbeil]

@wez the only thing not done right now is how to handle the vendored-openssl feature. With stable cargo, specifying the feature of a dependency as enabled will always include that feature, even if the dependency itself hasn't been included.

There was an issue about it rust-lang/cargo#3494, with a solution called weak dependency features merged in last year, but this feature is still only available on nightly cargo.

For now, I've added two features to enable vendored-openssl individually for ssh2 and libssh-rs.

[chipsenkbeil]

@wez if there's any early comments you might have, let me know :) I'm assuming everyone is taking a break throughout the holidays, so not expecting a full review until next year. This is one of the blockers for the next release of distant, so would love to get this rolling, though!

[wez]

Looks good, thanks!

[chipsenkbeil]

@wez just noticed that the weak feature functionality of cargo is now released: https://blog.rust-lang.org/2022/04/07/Rust-1.60.0.html#new-syntax-for-cargo-features.

If/when you bump to the new cargo version, you could collapse the old features above, removing the need for features like vendored-openssl-ssh2, vendored-openssl-libssh-rs. Instead, you'd have vendored-openssl = ["ssh2?/vendored-openssl", "libssh-rs?/vendored-openssl"].