Add site classification helper

[arunshenoy99]

1. Adds a SiteClassification helper class. Currently, for use in Implement Site Classification API (Backend Controller) wp-module-onboarding#220
2. Adds a HiiveWorker helper class.

Edit:

1. Adds SiteClassification, PrimaryType, SecondaryType and an abstract Types class for storing code common in both classes (also gives us an opportunity to add more types if needed in the future).
2. Falls back to a static JSON file in Data/Static/site-classification.json when it cannot fetch data from the worker, this static data is cached only for an hour, it also adds a static property to the data before caching and returning it.
3. PrimaryType and SecondaryType help in managing primary and secondary selections respectively, which includes validating, saving and fetching the data to/from the option.

[wpscholar]

@arunshenoy99 I don't have any background on the Site Classification functionality. Can you provide more details about what it is, how it works, why it needs to go in the data module, etc.?

[0aveRyan]

@wpscholar As you and I discussed on a call earlier this week after your last comment, this is an API is shared between multiple products. A number of months back in a PRESS1/2/4 sync we agreed it would go in here. Instead of returning an empty array we will commit a static/stale JSON file and return that data if the live API isn't returning a valid response and in a separate PR there will be shared getter/setters for those multiple products to classify a site. I will own updating that static JSON as the API data changes and in time the parser will use an action to deploy the live API and commit the static backup JSON into this repo.

[officiallygod]

LGTM