API: Trip Collaboration

Implement a set of API endpoints to allow users to collaborate on trip planning.

Add user collaboration endpoint:

• Develop an API endpoint to allow users to add other users as collaborators on a trip
• Implement specific permissions (edit, maintainer, etc)
• Add accounting details (percentage of bill coverage of specific user, etc)

Delete user collaborator endpoint:

• Implement an API endpoint to remove a user as a collaborator
• Updates bill splitting to exclude user