[Backend] Add an endpoint to get stats about authors.

[zergov]

From: https://github.com/godDeveloppeur/visualgraphgithub/blob/master/documentation/chart_data.md#commit-statistics-by-author

Commit Statistics by Author Key variables:

• name: The name of the author.
• nbCommits: The number of commits made by the author within the specified period (between the start and end dates). This represents the size of the bubble.
• commitsCategory: The number of commits made for each category of files. This represents the smaller bubbles inside the author’s bubble.
• ListCommitsCategory: A list containing commitsCategory objects for each category in the ModifiedCategory enum.

The color of the bubbles inside the author’s bubble corresponds to the file categories.

GET /repositories/123/authors/stats/commits_over_time

[
  {
    name: "Jonathan Lalande",
    commits_count: 69,
    commits_categories: {
      "DOCUMENTATION": 9,
      "SOURCE": 50,
      "CONFIG": 10,
    },
  }
]