Create a presigned upload URL for a marketplace demo image asset

Returns a presigned `PUT` URL for uploading an image asset into the marketplace demo's virtual folder in the publicly-accessible bucket. The URL is time-limited (default 2 hours) and may be reused until it expires; each successful `PUT` overwrites the object at the signed key. The URL is signed with the declared `size` as `Content-Length` and the declared `content_type` as `Content-Type`, so the client must upload exactly that many bytes (capped at 5000000 bytes) with a matching `Content-Type` header, otherwise the upload will be rejected. To intentionally overwrite an already-listed filename, request a new URL with `ignore_existing_image` set to `true` (or reuse an unexpired URL for that same key, size, and content type). Additionally, there is a 200 image per demo limit. The browser may show a limit of 100 images so a resized thumbnail can be stored too. See the boto3 docs for an example of uploading with `curl`: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/s3-presigned-urls.html **Required scope:** `air:marketplace_demo_write` (roles: `air_demo_manager`, `air_org_admin`, `air_user`) **Additional permissions:** - Only members of the owning organization, Air admins, or SREs may perform this action on this marketplace demo. - Applies whether or not the demo is published.