Support generation of single item embedded arrays

The `#to_hash` method on `Halibut::Core::Resource` never outputs embedded arrays that contain only one element. A couple of things were done to make that happen in a backwards compatible way.

* Adds an option to the `RelationMap` class so that it's `#to_hash` method will generate single item arrays when explicitly told to do so.
* Adds `Halibut::Core::Resource#add_embedded_resource` for populating embedded resources that should be rendered as arrays.
* Deprecates calling `Halibut::Core::Resource#embed_resource` for populating an embedded array. Reserves this method for providing a singular embedder resource. 

Hablibut::Builder has not been modified yet.