Implement EAV structure for product entity

[Jade-GG]

See #942, this is my alternative version, which includes attribute options and uses joins for attribute data & option values (but a relation for the options themselves).

Based on better benchmarks and cleaner relations, we have chosen to keep this approach as opposed to #942. I've taken some of the extra functionality from there and have implemented into this PR.

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With this PR we end up having 13 queries in total for products (regardless of how many products you query):

• 1 for the product itself
• 1 for the stock status
• 1 for the super attributes
• 2 for the category info (do we need this eager loaded?)
• 5 for each type of entity value
• 3 for the option relations, as 3 of the types (int, varchar, text) could have a custom option value, as far as I'm aware.

I've also made a small backend_model thing to parse data that may be in a weird format but I don't really know if this is a worthwhile thing to have abstracted like this. This is currently only used for comma separated values.

This also includes #944 currently, to add support for querying based on attributes. This is necessary to filter products based on the status attribute.

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Current to do list:

• Optimizations where possible
• Functionality that came from the scopes
  • Super attributes
  • Stock information
  • Upsells
  • Grouped products
  • Category information
  • Selecting for specific use cases, e.g. selectForProductPage
• Attributes
  • URL attribute
  • Images (all types)
  • Special price
  • Price (including configurable and grouped products)
  • Min sale qty
  • Breadcrumbs
• Relations
  • Product options
  • categoryProducts (is this necessary?)
  • reviewSummary (is this necessary?)
• Package updates
  • rapidez/reviews
  • ...finish this list
• Check the data that enters the indexer
• Optimize the indexer data because it is currently unacceptably slow
• Tier pricing
• Update and fix unit tests
• Compatibility with Runway
• Frontend
  • Update product page
  • ...finish this list
• Anything else I'm missing? Test everything and finish this list

[royduin]

Object caching is there by default? https://laravel.com/docs/master/eloquent-mutators#value-object-caching, but there is also a caching option: https://laravel.com/docs/master/eloquent-mutators#accessor-caching