Fixes #20253: GraphQL filter by contacts

[llamafilm]

Fixes: #20253

Define a custom filter method for contacts using the @strawberry_django.filter_field decorator.

[llamafilm]

I found a simpler / better solution — the contacts field should actually be a ContactAssignmentFilter. This way I can write more complex queries like this:

{
  site_list(
    filters: {
      contacts: {
        contact:{name:{contains:"Dwight"}}
        role:{name:{exact:"Operational"}}
        priority:PRIORITY_PRIMARY
      }
    }) {
    name
  }
}

[jeremystretch]

I think this is actually a bug, as it doesn't seem possible to filter by contacts at all using the current implementation of ContactFilterMixin. I suspect the mixin was intended to use ContactAssignmentFilter originally.

[jeremystretch]

Thanks @llamafilm!