New plugin: Duplicate product plugin

[martijnvdbrug]

For the Huidpraktijkshop project, we would like to have the ability to duplicate a product from the admin UI:

1. Admin navigates to product-A detail page
2. Admin clicks Duplicate
3. Admin is redirected to the duplicated product product-A-copy
4. All fields need to be duplicated, including:

• Facet values
• custom fields
• Custom field of type relation. E.g. we have 'related products' in the Huidpraktijkshop project, which would also need to be duplicated.

Implementation

My proposal (I'd like to hear your thoughts on this):

1. Create a Graphql mutation duplicateProduct(id: ID!): Product! that returns the duplicated product
2. The duplicated product is only created in the current channel, based on the CTX
3. An admin UI action bar button duplicate is added that calls the mutation duplicateProduct and redirects to the duplicated product detail page
4. E2E test: Please add an e2e test that includes a product with a plain text custom field, and a relation custom field, like related product

[martijnvdbrug]

This will be in core, not a new plugin. vendure-ecommerce/vendure#627