Limit admins to stores

[bandieramonte]

Is your feature request related to a problem? Please describe.
In some contexts, because of legal requirements, it is very important that the admins access only a limited set of stores in the dashboard, instead of all of them. For instance, when users create orders in different stores, right now any admin can see the orders of all stores. Legally it can be an issue in certain countries, when there is a store per country, and admins access sensitive data of all countries, when they should only be able to see the store of the country they belong to. The same applies to the catalog and customers of other stores.

Describe the solution you'd like
A good solution for this is that, in the same fashion that it is possible to create a new product and limit it to certain stores, it shall be possible to do exactly the same with customers. That is, it should be possible to create a new customer and limit it to certain specific stores. Then, such customer, who represents an admin, shall have access only to the sales/customers/catalog of the store it was given access to.

Describe alternatives you've considered
An alternative is that when admins check the list of orders that users have placed, via the Sales -> Orders menu item, it is possible to filter orders so that only orders of a certain store appear. However, this does not really satisfy the legal requirement since admins can anyways still access all orders.

Additional context
N/A

[bandieramonte]

@muratcakir I'm currently developing this feature since we need this kind of asap.
If you are interested, when finished, I can send a PR to merge into Smartstore's main branch.

[bandieramonte]

@muratcakir this feature can be fully implemented when merging in PR #1178