Implement changing assignments of users in Workspace

[nifadyev]

Add wrapper around PUT /organizations/{organization_id}/workspaces/{workspace_id}/assignments

Necessary steps:

• Check endpoint with all possible request body fields locally to find out if documentation is up-to-date
• Develop logic for endpoint based on similar methods
• Add necessary Pydantic schemas, update existing ones if necessary
• Add unit tests
• Add integration tests

Documentation