[Hivemind] Add fetch_pull_request_ids method to GithubPullRequestExtraction class

[cyri113]

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

This task consists of adding new methods to an existing class.

Given a repository id, using the pre-built neo4j connection, return an array of pull request ids associated with that repository

Tasks

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1. Write a public method fetch_pull_request_ids in GithubPullRequestExtraction
2. Write test cases

The class should have 100% coverage on code climate.

requires #99 .