SSH tunnel support

Description

This PR adds support for SSH Tunnel as requested many times in #205. It adds 6 new flags to the cli tool to perform the connection.

e.g.

dblab --host localhost --user postgres --pass password --schema public --ssl disable --port 5432 --driver postgres --limit 50 --ssh-host example.com --ssh-port 22 --ssh-user root --ssh-key my_ssh_key --ssh-key-pass password

This change only adds support for MySQL and Postgres, more to come later.

Fixes #205

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

A new docker compose file has been added to the development environment, adding a service called ssh, with ubuntu-sshd being its underlying base image to act as the ssh server to test the connection against.

Also, a mock ssh server has been added to the test suite, to validate the use of the private key file as authentication method.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings