feat: ssh dockerd connectivity

[matejvasek]

• 🎁 Add connectivity via SSH tunnel to remote docker daemons.
  Useful for instance when connecting to podman VM in macOS.

resolves #469

[knative-prow-robot]

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This pull-request has been approved by: matejvasek

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[matejvasek]

Just draft for now, need to decide what to do with https://github.com/matejvasek/sshdialer
Should I keep it as a library or just copy/paste code? what do you think?

Also the code from https://github.com/matejvasek/sshdialer will be used for buildpacks/pack#1259.

[matejvasek]

For now this will work with:

• pwd in ssh url e.g. ssh://user:pwd@host/path/to/podman.sock
• keys managed by ssh-agent
• keys with standard names in ~/.ssh/ that are not protected by passphrase

[matejvasek]

@zroubalik or @lkingland if you have some spare time could you please try this out on macOS?

[matejvasek]

I don't know why go mod tidy did this.

[knative-prow-robot]

@matejvasek: PR needs rebase.

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