Fix config file parsing for the ssh tunnel

Description

As reported in #270, dblab was not parsing the ssh tunnel configuration from the .dblab.yaml config file.

The reason was that the Database struct did not have fig struct tags for the ssh related fields. So, the fields reported on the README.md file were not being picked up by fig. The fix was actually adding the missing struct tags and proper testing for the ssh tunnel configuration.

Documentation files were updated too, to reflect the changes since some fields were not being documented.

Fixes #270

Type of change

Please delete options that are not relevant.

• 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 not work as expected)
• This change requires a documentation update

How Has This Been Tested?

The config package has a test suite, it was updated to validate the new fix.

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