[url-shortener] Custom Alias Support

[CSenshi]

Summary

Allow users to specify a custom alias for their short URLs, with collision detection and appropriate error handling.

Affected Area(s)

Apps:

• Url Shortener (apps/url-shortener)
• Web Crawler (apps/web-crawler)

Libraries:

• Shared (libs/shared)

Other:

• Other (please specify):

Motivation

Custom aliases are important for branding, memorability, and user experience. This feature will make the service more flexible and user-friendly.