PostgreSQL Integration improvments

[ZoranPandovski]

Description

This PR:

• Fixes https://github.com/mindsdb/mindsdb_private/issues/383 by checking if column value is Null or Nan and cast to 0
• Removes postgres_handler from pep8 ignored files and fix all warnings
• Add initial unit test for the handler
• Add missing README for the handler
• Improves logging and code comments

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

Verification Process

To ensure the changes are working as expected:

• Test Location: Use psql-improvments branch
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.

[MinuraPunchihewa]

Hey @ZoranPandovski,
Just a minor comment on the README, otherwise it looks good to me.

[ZoranPandovski]

@MinuraPunchihewa Done