Add "Scopes" object to request for Storefront API service

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Need to extend current Request to SF API to allow pass additional parameters to request (as customer group)

Current approach:
```

message ProductsGetRequest {
repeated string ids = 1;
string store = 2;
repeated string attribute_codes = 3;
// map params = 6; not implemented
}
```

Proposal approach:
```
message ProductsGetRequest {
repeated string ids = 1;
repeated string attribute_codes = 2;
Scope scope = 3;
}
message Scope {
string store = 1;
string customer_group = 2;
}
```

dev note: was changed in https://github.com/magento/catalog-storefront/commit/b2cd2af2cb095fb5c23a47ed304365b23bcf377f

Start by reviewing commit b2cd2af2cb095fb5c23a47ed304365b23bcf377f, which the issue notes as the development change, and compare it with the current Storefront API request definitions. Done means the request supports a Scope containing store and customer_group values for additional request parameters.

Written by the indexing model from the issue text.