Add Builder to simplify openapi document object construction

A regular C# builder pattern would help a lot in many scenarios.
Building an object now is very complex...

Builder pattern is also the "de facto" in Aspnetcore.. so would make sense to expose a similar api

var openApiDocumentBuilder = new OpenApiDocumentBuilder()
     .WithServers(...)
     .WithOperation(...)
     .WithOperation(...)
     .WithSchema(...);
//... etc


var openApiDocument = openApiDocumentBuilder.Build();
     

The issue names no files, tests, or entry points. Start by reviewing the existing OpenAPI document object model and current construction patterns, then clarify the builder's supported operations, schemas, and servers before implementation. Done would mean an agreed builder API can construct an equivalent OpenAPI document through Build().