GetDocumentInsider code should not overwrite up-to-date API descriptions

#### Is this a Bug or Feature request?
Enhancement that's part of #4896

#### Steps to reproduce (preferably a link to a GitHub repo with a repro project)
1. Create a test project with a `` to an (experimental) web site providing an `IDocumentProvider` service
2. Build the project
3. Notice build creates the expected API description file (aka Swagger or OpenAPI document)
4. Without deleting the above file, build the project again
5. Notice the timestamp of the description file changes and the client code is generated again

#### Description of the problem
The timestamp change is a problem because it extends the build time unnecessarily.

Problem does not occur when the document generation tool relies on `%(Uri)` metadata. That is, the download option works well already.

#### Version of `Microsoft.AspNetCore.Mvc` or `Microsoft.AspNetCore.App` or `Microsoft.AspNetCore.All`
See the [features/client.code.generation branch](https://github.com/aspnet/Mvc/tree/feature/client.code.generation) branch. That work is intended for the 2.2 Preview 3 milestone.