Add first GitHub entities and automatically fetch them for the leaderboard

[GODrums]

Motivation

To make our Leaderboard possible, we first have to fetch the required PRs / Comments data from Github's API and store them in the database, as well as make the data available through our own API.

Description

• Adds application-local.yml-support for the Github PAT
• Adds the database structure for Github's API/Webhook schema
• Provides a sample runner to store Hephaestus`s data in the database
• Provides a Github API Client + GraphQL Client as fallback

Next up for the leaderboard (in other PRs):

• Webhook integrations (NATS)
• More endpoints depending on which data we need in our front-end

Screenshots (if applicable)

Checklist

General

• [x] PR title is clear and descriptive
• [x] PR description explains the purpose and changes
• [x] Code follows project coding standards
• [x] Self-review of the code has been done
• [x] Changes have been tested locally
• [ ] Screenshots have been attached (if applicable)
• [x] Documentation has been updated (if applicable)

Server (if applicable)

• [x] Code is performant and follows best practices
• [x] No security vulnerabilities introduced
• [x] Proper error handling has been implemented
• [ ] Added tests for new functionality
• [ ] Changes have been tested in different environments (if applicable)

[FelixTJDietrich]

I think the code looks good now, maybe you can take a quick look at ResponseEntity and the returned NotFoundException. Also you need to update the angular API client library, there is a command that you need to run in package.json. Afterwards we can merge this PR :)