Generate a MediaLit upload signature

Returns a short-lived MediaLit signature and endpoint. Upload image files directly to `${endpoint}/media/create/resumable` with the signature in the `x-medialit-signature` header.

curl -X POST "https://api.sendlit.dev/media/presigned"

{
  "signature": "string",
  "endpoint": "string"
}

{
  "error": "string"
}