Enable request transforms to reject requests

[Tratcher]

Fixes #1701 @leastprivilege

If a request transform encounters a problem right now the only thing it can do is throw. A few customers have asked how to generate a response and exit gracefully. Response transforms can already do this by returning false to prevent us from proxying the body.

• Request/response validators #1701
• Mandatory Request header validation #1670
• What is the correct way to return an error/status code in `HttpTransformer`? #1724

HttpTransformer.TransformRequestAsync has a signature that can't return any additional information without an API breaking change. Instead, we can detect if a transform generated a response by checking HttpResponse.StatusCode or HasStarted. In this case we'll stop trying to proxy the request and exit gracefully. If someone had modified the StatusCode before this change it would have been overwritten when proxying the response. If they had started the response it would have failed when trying to proxy the response later.

There's a series of related requests that I checked if could be solved in the same way but decided against it. I think these can be handled at the config and routing layer with alternate endpoints, there's no need for them to execute the proxy pipeline / load balancing, session affinity, etc. I'll send a separate PR for those.

• No way to reject a route #1893
• Cluster "Action" for given routes #1868
• Proxy can deliver static files directly #187

TODO: Update docs