feat: add progress tracking to PostgreSQL result backend

[long2ice]

• Introduced a new progress column to the result table to store task progress.
• Implemented set_progress and get_progress methods for updating and retrieving task progress in the PostgreSQL backend.

Description

Brief description of the changes in this PR.

Type of Change

• 🐛 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)
• 📚 Documentation update
• 🔧 Maintenance (dependency updates, CI improvements, etc.)

Changes Made

• Change 1
• Change 2
• Change 3

Testing

• 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 tested with multiple PostgreSQL drivers (if applicable)

Test Configuration

• Python version(s):
• PostgreSQL version:
• Driver(s) tested:
• Operating System:

Performance Impact

• No performance impact
• Positive performance impact
• Negative performance impact (please describe)

Performance details:

Documentation

• I have updated the documentation accordingly
• I have updated the README if needed
• I have added docstrings to new functions/classes

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
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published in downstream modules

Related Issues

Closes #(issue number)

Screenshots (if applicable)

Additional Notes

Any additional information that reviewers should know about this PR.

[ans-artal]

@long2ice
I think saving the progress under the same key as the results and making the result column nullable=True will affect the behavior of get_result.

With this change, once progress is set, get_result stops raising ResultIsMissingError and returns None instead.

Also, if someone sets keep_results=False and calls get_result, the progress gets deleted even if the results have not been set yet.

====

Maybe you could handle it like the official Redis backend:
https://github.com/taskiq-python/taskiq-redis/blob/main/taskiq_redis/redis_backend.py

It reuses the result table but appends a PROGRESS_KEY_SUFFIX to the task UUID.

[ans-artal]

Maybe :
#23