[Hivemind] Refactor Github pull request extraction functions to object orientated code

[cyri113]

Part of the Github Vectorize (Summary). Please read this document before starting.

This task consists or refactoring a part of the code from functional programming to object-orientated code.

The following changes should be implemented in dags/hivemind_etl_helpers/src/db/github/extract.

Tasks

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1. Create a class GithubPullRequestExtraction
2. Migrate fetch_raw_pull_requests to a private method _fetch_raw_pull_requests in GithubPullRequestExtraction
3. Migrate the fetch_pull_requests to a private method fetch_pull_requests in GithubPullRequestExtraction
4. Make the class GithubExtraction inherit GithubPullRequestExtraction
5. Update the code base to use the GithubPullRequestExtraction.fetch_pull_requests instead of the fetch_pull_requests
6. Update the test cases

Note: file coverage should be 100%.