Add optional environment variables to configure Privacy Center and Google Tag Manager container for the "Cookie House" sample app

[NevilleS]

Is your feature request related to a specific problem?

For testing the fides.js Google Tag Manager (GTM) integration (aka window.Fides.gtm()), it'd be useful to have GTM set up in the sample application for manual & automated testing. This is similar to #3277 - the more "realistic" we can make the Cookie House, the better it will be as a testbed for us.

Similarly, we currently have the Privacy Center hardcoded to localhost, whereas that should be configurable for when we deploy this image.

Describe the solution you'd like

I'd like an optional environment variable, FIDES_SAMPLE_APP__GOOGLE_TAG_MANAGER_CONTAINER_ID that defaults to null. If set, we should include the tag manager <script> in the head of the sample app (see https://developers.google.com/tag-platform/tag-manager/web)

Additionally add FIDES_SAMPLE_APP__PRIVACY_CENTER_URL for the "Do Not Sell..." link and fetching fides.js

[NevilleS]

Hah, good news: this is literally already implemented here: #2949, just needs to be brought up-to-date with the clients refactor.

[Roger-Ethyca]

moving to done