[357.6] GetVariantsMatch service

[bricht]

Subtask of #357

Depends on #331

Steps
Implement GetVariantsMatch service.

message OptionSelectionRequest
{
string store = 1;
// array of option_values with the following format parent_id:option_id/optionValue.uid
// @see ProductValue.option_values
repeated string values = 2;
}

service VariantService {
  rpc GetVariantsMatch (OptionSelectionRequest) returns (ProductVariantResponse) {}
}

see https://github.com/magento/catalog-storefront/blob/develop/dev/docs/designDocuments/Configurable-products-option-variants.md for technical details.
cover with API-functional tests

[m2-assistant]

Hi @bricht. Thank you for your report.
To help us process this issue please make sure that you provided sufficient information.

Please, add a comment to assign the issue: @magento I am working on this

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[RuslanKostiv1]

@bricht , we already have an issue for GetVariantsMatch
#359 , this one can be closed as duplicate