Create avatar upload

Start the avatar image upload workflow by requesting an upload target.

This endpoint is step 1 of a multi-step process and does not update the account avatar on its own.

Avatar upload workflow:

1. Call this endpoint to receive an upload payload, including image identifier and upload URL.
2. Upload the binary image directly to the returned URL.
3. Call PATCH /user/account/avatar with the returned image identifier to finalize.

The avatar is only applied after successful finalization.