[API Review] Rename & evaluate properties in ReadResult

[RachitMalik12]

Requester: @captainsafia

1. Evaluate rename of properties in ReadResult to avoid redundancy in naming:
   Change OpenAPIDocument -> Document and OpenAPIDiagnostic -> Error
2. Evaluate if the properties in ReadResult need setters.
3. OpenAPIDocument should be nullable.

[MaggieKimani1]

I think we can rename OpenApiDiagnostic to Diagnostic since it already contains an errors object within, in addition to a SpecificationVersion object. It would make more sense to have Diagnostic.Errors and Diagnostic.SpecificationVersion instead of Error.Errors

[RachitMalik12]

@MaggieKimani1 I am fine with that.