Add support for GitHub Pull Request entities

[arkid15r]

Describe the solution you'd like

We need to track open pull requests (PRs) for repositories. To achieve this, we will:

1. Create a PullRequest Model

   • Add a new PullRequest model under github/models/pull_request.py to reflect GitHub’s pull request structure.
   • See other similar models for better implementation.
   • Reference the GitHub API documentation for PR details: GitHub REST API - Get a Pull Request

2. Update sync_repository to Fetch and Store PRs

   • Retrieve open PRs and store them in the database.
   • Ensure that the status of closed PRs is updated to match their current state on GitHub.
   • Use repository.get_pulls() from PyGithub to retrieve PRs.
   • Set appropriate state, sort, and direction parameters for efficient data fetching.
   • Reference PyGithub documentation: Repository.get_pulls().

[arkid15r]

Nikhil Patil (space-techy) wants to implement this one but I'm not sure about the username to tag.

[Raghavg18]

@arkid15r can u assign this to me?

[Yashgupta9330]

@arkid15r I want to work on these issue can you assign me this issue

[space-techy]

Hello @arkid15r Yes I am working on it!
Sorry for the late reply!
Also this is my username : @space-techy