Use name "Swagger" more and switch to use IOptions<T> for middleware

- move `PostProcess` document from `SwaggerDocumentSettings` to `SwaggerMiddlewareOptions`
  - adjust `SwaggerDocumentProvider` to match, breaking some `IDocumentProvider` scenarios (!!)
- rename a few classes to use "Swagger" more
- split extensions into type-specific classes with appropriate namespaces
- fix `SwaggerMiddlewareOptions.SwaggerRoute` being invalid from a Microsoft.AspNetCore.Routing perspective
- change `SwaggerBuilderExtensions` to support creating documents with `SchemaType.OpenApi3` default
- change `SwaggerApplicationBuilderExtensions` to
  - split `UseSwagger(...)` from `UseSwaggerReDocUi(...)` and `UseSwaggerUi4(...)`
  - add UI middleware supporting a single `documentName`

"Reviewers:" comments ask make suggestions for future improvements / extensions and ask for better names

src/NSwag.AspNetCore/DependencyInjection/IOpenApiBuilder.cs → src/NSwag.AspNetCore/DependencyInjection/ISwaggerBuilder.cs

@@ -1,19 +1,17 @@
 //-----------------------------------------------------------------------
-// <copyright file="IOpenApiBuilder.cs" company="NSwag">
+// <copyright file="ISwaggerBuilder.cs" company="NSwag">
 //     Copyright (c) Rico Suter. All rights reserved.
 // </copyright>
 // <license>https://github.com/NSwag/NSwag/blob/master/LICENSE.md</license>
 // <author>Rico Suter, mail@rsuter.com</author>
 //-----------------------------------------------------------------------
 
-using Microsoft.Extensions.DependencyInjection;
-
-namespace NSwag.AspNetCore.DependencyInjection
+namespace Microsoft.Extensions.DependencyInjection
 {
     /// <summary>
-    /// An interface for configuring Open Api documents.
+    /// An interface for configuring Swagger and Open API documents.
     /// </summary>
-    public interface IOpenApiBuilder
+    public interface ISwaggerBuilder
     {
         /// <summary>
         /// Gets the <see cref="IServiceCollection"/>.