fix/immutable reference

[baywet]

makes document for references optional. Cleans up internal constructors for references, makes references immutable to avoid side effects

related #1998

[sonarqubecloud]

Quality Gate failed

Failed conditions
70.3% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[baywet]

@captainsafia this is where I wanted to get:

• The proxy design pattern is properly implemented, one interface, two implementations. The consumers can use type assertions while reading a document to know what's mutable, what's not.
• All reference types now offer a single public constructor, which only requires the reference ID but also accepts document, external resource. This way creating a document from the DOM remains simple/doesn't require carrying document references all over the place.
• The target property is a pure function (no more backing field with potential side effects)
• The reference property is immutable (no more side effects)
• The properties of the reference type are also immutable (no more side effects)

The only drawback I can see is having to call RegisterComponents and/or SetReferenceHostDocument on the OpenAPIDocument based on what the consumer is doing. But we could "hide that" by calling those methods before serialization.

We'll try to release soon, in fact #2115 is already up so you can test that ASAP. I've noticed a much better developer experience and reliability on my end in kiota, openapi.net.odata, plugins, and API manifest libs. (linking locally)