[Backend] Add an endpoint to get stats about commits over time.

From: https://github.com/godDeveloppeur/visualgraphgithub/blob/master/documentation/chart_data.md#change-scope-in-the-repository

Key variables:

date: The date of the commit. This represents the X-axis.
nbCodeLinesModified: The number of lines of code modified between the start and end dates. This represents the Y-axis.
nbModifiedFiles: The number of modified files in the commit (commit scope). This represents the size of the bubble.
typeOfFiles: The predominant type of files modified (e.g., Configuration, Test, Documentation, Production). This represents the color of the bubble.

The endpoint should return something like:

GET /repositories/123/commits/stats/commits_over_time

[
  { 
    date: "2010-01-01", 
    modified_lines: 49, 
    modified_files: 13,
    categories: "DOCUMENTATION"     # <--- not sure how to do this yet, but yolo  
  }
]

visevol / GithubVisualisation

[Backend] Add an endpoint to get stats about commits over time. #50