Update discord_cdn.ex

[MrShadowDev]

1. Added type specifications to functions for clarity and readability.
2. Extracted the logic for constructing the constructed CDN URL into a separate function for better separation of concerns.
3. Used pattern matching with a guard clause in the get_proxied_avatar/4 function instead of an if statement for readability.
4. Used the pipe operator to simplify the code and make it more readable.