Flatten namespace hierarcy

[darrelmiller]

Is your feature request related to a problem? Please describe.
There are a set of namespaces that exist in the OpenAPI.Net library that have been created due to the organization of class files by folder. However, there is no value added by having matching namespaces. It requires the knowledge and use of "using" to bring in these namespaces.

Describe the solution you'd like
Align all most classes with a single top level namespace

Change the following namespaces to Microsoft.OpenApi:

Microsoft.OpenApi.Attributes
Microsoft.OpenApi.Exceptions
Microsoft.OpenApi.Helpers
Microsoft.OpenApi.Interfaces
Microsoft.OpenApi.Models
Microsoft.OpenApi.Models.Interfaces
Microsoft.OpenApi.Models.References
Microsoft.OpenApi.Properties

Change the following namespaces to Microsoft.OpenApi.Reader. It is useful to be in a different namespace because not everyone needs to read documents:

Microsoft.OpenApi.Reader.ParseNodes
Microsoft.OpenApi.Reader.Services

Leave these namespaces as they allow consistent class names across the versions

Microsoft.OpenApi.Reader.V2
Microsoft.OpenApi.Reader.V3
Microsoft.OpenApi.Reader.V31

Microsoft.OpenApi.MicrosoftExtensions