Do not overwrite existing IConfiguration

[waynebrantley]

host.json is read and used by the runtime. Extension settings are applied at startup for things like queue times, etc.

  "extensions": {
    "queues": {
      "maxPollingInterval": "00:00:02",
      "visibilityTimeout": "00:00:30",
      "batchSize": 16,
      "maxDequeueCount": 5,
      "newBatchThreshold": 8
    }
  } 

However, when your code overwrites the IConfiguration all these settings get removed.
This PR leverages an overload designed to alter the configuration by adding app settings.
Additionally, there are new extensions to assist when needing the environment name (like when setting up logging, etc).
Using context, was able to make useLogger not do a manual lookup in IServiceCollection

Once these are in place, you still get all the features you were intending on adding plus all the built in ones.

[junalmeida]

@waynebrantley
I loved your changes, I will try to get some time to make a test this weekend.
I have one question for you, is your assumption based on the very latest version of the library? I'm asking because it indeed removed all previous host.json configs in previous version, but I've made changes to avoid that on the very latest version. If you are using the latest version it means that didn't work for you, right?
In any case your code seems better, if it works I will merge.

[waynebrantley]

I had not updated to your very latest version. I looked at your changes and it did not seem that it would help with this situation. Thanks for considering!