[6.x] Query Builders

[duncanmcclean]

This pull request implements query builders for products, orders & customers.

This means we can use the same query across multiple repositories which was previously not possible as you had to implement the same-ish query two/three times depending on the repository in use.

The query builders extend the EntryQueryBuilder and EloquentQueryBuilder that are built into Statamic.

This PR shouldn't make much of a difference to end-developers, apart from the fact Product::all()/Customer::all()/Order::all() will now return Simple Commerce objects, rather than entries/Eloquent models.

This PR also improves the state of testing around the various product/order/customer repositories.

To Do

• [x] Implement query builders for orders
• [x] Implement query builders for customers
• [x] Implement query builders for products
• [x] Add tests around the various query builders
• [x] Refactor existing queries
• [x] Make sure id column in Eloquent queries actually queries whatever the model key is.
• [x] Resolve failing tests

[github-actions[bot]]

Released as part of v6.0.0.