Implement /v2 endpoints for user and access management #227

As a followup after https://github.com/MerginMaps/roadmap/issues/60

Let's implement the new endpoints to allow users manage accounts and access in a workspace.

Py-API methods

These will become deprecated (in the 2nd phase), alongside their CLI commands:

• add_user_permissions_to_project
• remove_user_permissions_from_project

We want to add these:

User creation

• create_user

Projects API

• list_project_collaborators
• add_project_collaborator
• update_project_collaborator - (user_id: int) -> void
• remove_project_collaborator

Workspaces API

• list_workspace_members
• get_workspace_member
• add_workspace_member
• update_workspace_member
• remove_workspace_member

Process:

1. implement py-client API + tests
2. implement CLI (Adjust CLI to reflect /v2 management endpoints & subgroups #229)