Optionally log RpcEvent (strictly for development)

As we mention [here in the wiki](https://github.com/Azure/azure-functions-host/wiki/Authoring-&-Testing-Language-Extensions#logs), we should be able to set a host.json configuration (likely in combination with an app setting/environment variable) to allow for verbose debug logging. This debug logging should be strictly for authors of Azure Functions language extensions

Read the wiki's Authoring & Testing Language Extensions logs section first, then trace where RpcEvent logging and host.json configuration are handled. Check how app settings or environment variables are read. Done means development-only verbose RpcEvent logging can be enabled through the proposed configuration without affecting normal operation.

Written by the indexing model from the issue text.