add endpoint existence check and update functionality by slug

[shubham3121]

This pull request adds new functionality for managing endpoints by their slug, including checking for existence and updating endpoints, and introduces supporting methods in the service layer. The main changes are grouped into API additions and service logic updates.

API additions:

• Added GET /{endpoint_slug}/exists endpoint to check if an endpoint exists for the current user by slug.
• Added PATCH /{endpoint_slug} endpoint to update an endpoint by slug for the current user.

Service logic updates:

• Implemented endpoint_exists_for_user method in EndpointService to check endpoint existence by user and slug.
• Added update_endpoint_by_slug method in EndpointService to update endpoints by slug, including contributor validation and permission checks.