Multiple environments for host.json

#### What problem would the feature you're requesting solve? Please describe.
When debugging a service bus triggered Azure Function locally I often have multiple messages handled simultaneously. This troubles the debugging process. As a result of that I fix it with a trick to handle only 1 service bus message at the time by adding the following to host.json
```json
{
"version": "2.0",
"extensions": {
"serviceBus": {
"messageHandlerOptions": {
"maxConcurrentCalls": 1
}
}
}
}
```

#### Describe the solution you'd like'
However, either I have to add the host.json file to the .gitignore file, which I don't like. Or I have to revert my changes every time I commit. I would be nice to have a solution just like with `appsettings.development.json`. In this way I never accidentally commit the `maxConcurrentCalls` setting and kill the performance 😂

Start by reviewing how host.json is loaded and compare the requested behavior with appsettings.development.json. Done means developers can use environment-specific local host settings, such as maxConcurrentCalls, without changing or ignoring the committed host.json.

Written by the indexing model from the issue text.