Option to use Hugo Extended

There is functionality in the static-web-apps GitHub Action to select which version of Hugo you would like to use. However, there is not a property to determine whether you would like Hugo or Hugo Extended.

Hugo Extended is used if you plan to use SASS/SCSS in your Hugo theme. I've made a custom theme for my site, so have this as a requirement (per Hugo Community - https://discourse.gohugo.io/t/should-i-use-hugo-extended-for-a-new-hugo-project/13954/6)

The workaround (thanks @anthonychu), was to build using the steps I'm already using to compile the files for deployment to Azure Storage's Static Sites, but in the static-web-apps GH action set the app_location to the build output from the previous step, and set skip_app_build to true.

Here are some examples of my workflows:

- [Building and Deploying to Azure Storage Static Sites](https://github.com/chrisreddington/cloudwithchris.com/blob/master/.github/workflows/master.yaml)
- [Building and Deploying to Azure Static Web Apps](https://github.com/chrisreddington/cloudwithchris.com/blob/master/.github/workflows/azure-static-web-apps-gray-beach-021c69b03.yml)