Add pagination to product variants.

[bricht]

Need to add pagination to product variants and remove current cap of 5000 variants per request.

Modify the GetProductVariants service and the elasticsearch query logic to support pagination.

service VariantSearchService {
    rpc GetProductVariants (ProductVariantRequest) returns (ProductVariantResponse);
}```

See Magento\StorefrontStorage\Model\Client\ElasticsearchQuery

[m2-assistant]

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[mslabko]

@magento export issue to Jira project SFAPP as Story

[github-jira-sync-bot]

✅ Jira issue (https://jira.corp.magento.com/browse/SFAPP-313) is successfully created for this issue.