Allow the `x-koha-embed`literal in OpenAPI object definitions

On writing a small patch for bug 34333, I noticed our QA tools reject the presence of x-koha-embed if it doesn't have the following form:

name: x-koha-embed

We do that to detect the presence of top-level occurences of it at the path level. But in this case I was trying to mention on the object spec that the added attribute is to be requested to the API through an x-koha-embed header. This style is already used in the codebase:

$ cd api/v1/swagger/definitions
$ git grep embed
ill_request.yaml:    description: The linked biblio object (x-koha-embed)
ill_request.yaml:    description: The linked comment objects (x-koha-embed)
ill_request.yaml:    description: The linked comment objects count (x-koha-embed)
ill_request.yaml:    description: The linked extended ill request attributes (x-koha-embed)
ill_request.yaml:    description: The linked library object (x-koha-embed)
ill_request.yaml:    description: The id_prefix of the request (x-koha-embed)
ill_request.yaml:    description: The linked patron object (x-koha-embed)
ill_request.yaml:    description: Expanded coded fiels (x-koha-embed)
item_group.yaml:    description: A list of items that belong to the volume (x-koha-embed)

I think it is useful to document those attributes like that so I propose to lift this requirement so the literal x-koha-embedis accepted on a description.