Add ecdsa cert-based auth via SSH Agent

[murkvin]

Context

Users of "Open Remote - SSH" have requested support for certificate-based authentication. This feature request is currently tracked in jeanp413/open-remote-ssh#95.

Currently "Open Remote - SSH" fails to authenticate to hosts that require certificate-based authentication. This is, primarily, because ssh2 does not implement certificate-based authentication.

This issue is called out across multiple Github Issues and Pull Requests upstream, including, but not limited to:

1. Feature Request: support certificate authentication.  mscdex/ssh2#551
2. SSH client with authentication by signed certificates does not work with SSH_AGENT mscdex/ssh2#910
3. How to connect with a key signed by a certificate authority? mscdex/ssh2#858
4. Client: Allow explicitely specifying a publicKey mscdex/ssh2#808

Approach

Meta: This PR is (mostly) a clone of the one currently sitting upstream in mscdex#1440. I have cloned this here since "Open Remote - SSH" is built on top of this ssh2 fork.

This code change adds additional key parsing logic that, when other methods fail, will attempt to parse the key as a certificate. During the authentication flow, these certificate key types are then mapped to the corresponding signature type.

On top of the original cert types, I added several non vendor-extension cert types called out in the internet draft: https://www.ietf.org/archive/id/draft-miller-ssh-cert-01.html.

Importantly, this only works via an SSH agent (more on that later). This lightweight implementation serves to unblock folks who otherwise would be unable to connect to their hosts. In order to make use of certificates only available on disk, more significant changes would need to be made to the library to prevent public keys from being overwritten by the library-generated keys (depending on what is available on the machine).

NB: This CR is targeted toward addressing common ecdsa keys.

User Experience

In order to connect to hosts requiring certificate-based authentication, users will need to add their certificate to their running ssh agent by running, for example ssh-add. If the certificate expires, then they will need to re-add the certificate to their running agent.

Testing

I tested these changes by linking them as a dependency to "Open Remote - SSH" and connecting to a host that requires (ECDSA) certificate-based authentication.

Follow-up steps

• A version bump and new release of Open Remote - SSH
• Deployment out to Open - VSX

Relevant links:

• https://datatracker.ietf.org/doc/html/rfc4252
• https://www.rfc-editor.org/rfc/rfc5656
• https://www.openssh.com/txt/release-8.4
• https://www.ietf.org/archive/id/draft-miller-ssh-cert-01.html
• https://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-19