Add --server-ssh-key for join command

[JerryKhw]

Description

Added the --server-ssh-key flag to the join command to support the ssh key for the existing k3s server.
If the flag does not exist, we have worked to ensure that it operates as the existing function.

Why do you need this?

• I have raised an issue to propose this change (required)

Was raised by someone else. #404

How Has This Been Tested?

./k3sup join \
  --ip $HA_IP \
  --user $HA_USER \
  --server \
  --server-ip $SERVER_IP \
  --server-user $SERVER_USER \
  --server-ssh-key $SERVER_SSH_KEY_PATH \
  --ssh-key $SSH_KEY_PATH

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.