Multiple currency enablement

### User Story

As a business user, I want to give website users the ability to change the currency displayed on the e-commerce.

### Description & Motivation

Current version of CIF doesn't allow to display a currency selection with currencies [configured](https://devdocs.magento.com/guides/v2.4/graphql/queries/directory-currency.html) on Magento
### Deliverables

CIF must be able to pass to each Magento call the header Content-Currency as specified in the [documentation](https://devdocs.magento.com/guides/v2.4/graphql/send-request.html):

Content-Currency | A valid currency code, such as USD | This header is required only if the currency is not the store view’s default currency.
-- | -- | --

### Acceptance Criteria

- website user must be able to select the desired currency for e-commerce
- this currency value must be used in Magento calls
- graphql calls must support the header Content-Currency to use the value selected by end user

No implementation files or tests are named. Start by tracing CIF's existing Magento GraphQL calls and the currency-selection entry point, then consult the linked Magento currency and Content-Currency documentation; done means users can select a currency and that value is sent in the header for each relevant call.

Written by the indexing model from the issue text.