move global struct to anonymous struct

[chrismarget-j]

The blueprints struct in apstra/data_source_blueprints.go is only used in a single spot.

type blueprints struct {
	Ids       types.Set    `tfsdk:"ids"`
	RefDesign types.String `tfsdk:"reference_design"`
}

Move the global definition to an anonymous struct instantiation at the point of use.

https://github.com/Juniper/terraform-provider-apstra/blob/2ddac8d857da05d275eb7ce2fdd8bec9e5e20d1f/apstra/data_source_blueprints.go#L92C1-L95C2