Add --ssh-jump CLI argument

[rolandwalker]

Description

Add --ssh-jump CLI argument, restoring functionality removed in v2.0.0, under a different interface, and with a different internal implementation.

In the new interface, there is a single argument --ssh-jump, which accepts values in the form user@host:port. Cleartext SSH passwords on the CLI are no longer accepted. The user will be prompted for a password if the connection is not otherwise configured.

Deprecated arguments such as --ssh-host now refer to --ssh-jump.

Remote sockets are accepted. The user should give the MySQL endpoint exactly as it would be accepted from the jump host.

A keyring field is added for the ssh-jump value, if any, but the field is not appended unless present, so existing keyring values are not invalidated.

The internals of the new implementation are completely different: instead of using complex Python libraries (paramiko/sshtunnel) we simply open a tunnel using the ssh executable in a thread. This is more robust in a few ways: paramiko for example is not capable of reading nontrivial SSH config files.

A new [ssh] section is added to myclirc with some recommended CLI arguments to SSH when establishing tunnels.

Fixes #1960.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .