Add AI generation for the initial PR description (#38)

## Summary

This pull request introduces the capability to automatically generate
initial descriptions for new pull requests using AI. The main changes
include the integration of AI services to process the diff and files
affected by a commit and generate a human-readable summary that fits
into the existing pull request template.

Significant code modifications include:
```python
if new_pr_body is None:
    new_pr_body = self.process_build_pr_body_with_ai(
        commit_hash=commit.hash,
    )
```
This snippet ensures that if no PR body is provided manually, the system
attempts to generate one using AI, based on the current commit's hash.

## How was this tested?

It's an early prototype yet. Dogfooding.

## PRs in the Stack
- ➡ #38

(The stack is managed by
[git-grok](https://github.com/dimikot/git-grok).)

Author: dimikot