implemented return artists top tracks

[bryjjch]

Motivation and Context

Gives us the top tracks of an artist.

Your Changes

Added a get_artist_top_tracks method in the SpotifyAPI.

Description:

Type of change (select all that apply):

• [X] New feature (non-breaking change which adds functionality)

Checklist

• [X] I have performed a self-review of my own code.
• [X] I have added tests for my changes.
• [X] Coverage tests
• [X] Coding style (conforms to Google Java Style Guide)
• [X] Especially follows section 7 (Javadoc is present for every public class, public or protected member)
• [X] Especially follows section 4.6 (Multiple consecutive blank lines are not encouraged)
• [X] Correctness (follows CA and SOLID, is a reasonable and not overly complicated solution, etc.)
• [X] Description of pull request is clear, consise, and reasonably detailed