Automatically generate OpenAPI specification based on functions

**Describe the issue**
At the moment you have to manually add attribute classes on top of the FunctionName decorator to enable OpenAPI metadata and make the method visible in the swagger-ui.

**Expected behavior**
OpenAPI specification should be generated automatically based on Functions with HTTP-Trigger.

**Additional context**
E.g. Swashbuckle for ASP.NET Core has the [Swagger-Generator](https://www.nuget.org/packages/Swashbuckle.AspNetCore.SwaggerGen/) which does exaclty that.

Start by reviewing how the extension currently uses attribute classes on FunctionName-decorated methods to produce OpenAPI metadata and expose operations in swagger-ui. Compare the requested automatic generation for HTTP-triggered functions with the linked Swashbuckle Swagger Generator. Done means HTTP-triggered functions appear in the generated OpenAPI specification without manually added metadata attributes.

Written by the indexing model from the issue text.