Project members management via public API

[tmarenko]

Describe the feature or potential improvement

Introduce public API for managing project members to allow automated addition and removal of members.

Additional information

Currently, adding or removing members is restricted to manual processes via UI. By enabling API-based member management, we can automate these processes to ensure quick and secure adjustments to project access.

We avoid the use of LANGFUSE_DEFAULT_PROJECT_ID since it's crucial for maintaining the security integrity of projects (by default, new users are assigned to an empty project).

[tmarenko]

any chance that someone will take a look?