Add SSH key management to CLI

[nicolai86]

What does this PR do?

This PR adds three new CLI commands enabling users to manage their SSH keys for lightning cloud and BYOC.
There will be a separate follow-up PR to document this functionality properly.
As SSH support is disabled via a feature flag these can be split up.

This was discussed with @awaelchli who in turn also touched base with Will:

lightning add ssh-key --public-key /path/to/key.pub
lightning add ssh-key --public-key "ssh-rsa ..." # in case you use a hardware key
lightning list ssh-keys
lightning remove ssh-key <key-id>

Fixes #<issue_number>

Does your PR introduce any breaking changes? If yes, please list them.

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cc @Borda

[manskx]

Looks great !