[Hivemind] Create GithubSummaryGenerator class

[cyri113]

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

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

Tasks

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1. Create a class GithubSummaryGenerator
2. Inherit from SummaryBase. See example.
3. Create a public method generate_summary. Given an array of llama-index documents, using the _get_summary class and the query, return a string summary.
4. Create the required test cases

Note: file coverage should be 100%.