Creates a new session (link for new user interaction with Avaturn UI) and returns it's url and id.

There are 3 types of session:

• create_or_edit_existing: starts with avatar dashboard if user has any avatars or with avatar creation flow if user has no avatars;
• create: starts with avatar creation flow regardless avatar existence;
• edit_existing: starts with editor for avatar with provided id.

Send resulting url to your frontend and open it in webview or iframe. Use session_id to distinguish webhook events between multiple sessions.

NOTE: The resulting session url itself is considered implementation detail, and its structure is not guaranteed to be backward-compatible.