Add support for token exchanger in Express

[bakura10]

Overview

Hi,

Shopify recently introduced a new way to exchange tokens (https://shopify.dev/changelog/new-oauth2-token-exchange-api-shopify-managed-install-authorization-flows-available) and reduce flickering and redirect (which has been a common problem for OAuth process).

Unfortunately, while the feature has been added to the SDK (https://github.com/Shopify/shopify-api-js/blob/main/packages/shopify-api/docs/guides/oauth.md#token-exchange) it does not seem to be usable yet in Express app (only Remix app).

Migrating all apps to Remix is not an easy task, so it would be nice to have this supported in the Express app as well to improve the overall user experience for existing merchants.

Thanks.

[matteodepalo]

Hi @bakura10, thank you for opening this issue. This improvement to Express is not in our list of priorities at the moment, but if anybody wants to work on a PR to implement it we'll be happy to review and merge it.

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