Move filters for the products query into a separate input type

[janus-reith]

Issue Description

The products query currently defines all possible filters toplevel, making it impossible to extend:
https://github.com/reactioncommerce/api-plugin-products/blob/0be4a8e31dab70475431f892b66e8ce8c3f4f6e1/src/schemas/product.graphql#L615

In comparison, this is what it looks like for the orders query:
https://github.com/reactioncommerce/api-plugin-orders/blob/419872d845d8ed24b424510fcc7e645db85cef11/src/schemas/schema.graphql#L17

OrderFilterInput can be extended easily, custom plugin logic would only need to extend the schema.

In general I believe there still is some way to go to unify these schemas and find a more generic interface for CRUD operations.

Steps to Reproduce

1. Try to add a filter for the products filter
2. Realize your only options are forking the plugin, or creating a custom query, duplicating the logic of the products query.

Possible Solution

Move all filters into an ProductFilterInput type.

Versions

trunk