[Hivemind] Add fetch_obj_date method to GithubObjectExtraction class

[cyri113]

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

Given a list of pr and issue ids, using the psql connector, query PostgreSQL to retrieve the PR and issues’ saved date on summaries data, return a dictionary with the id -> date (the date can be found in the metadata)

Tasks

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1. Create a class GithubObjectExtraction
2. Make the class GithubExtraction inherit GithubObjectExtraction
3. Write a public method fetch_obj_date in GithubObjectExtraction
4. Write test cases

The class should have 100% coverage on code climate.