Add 2 new env vars: Admin token & store

[isaacroldan]

WHY are these changes introduced?

We need a way to execute some commands (that need Admin API access) without any interaction.
With this PR we introduce two new Environment variables that allow us to set a store and a token (with access to that store).

If provided, a command like theme pull will ignore any cached session/store and use the ones from the environment.

WHAT is this pull request doing?

• Add SHOPIFY_CLI_ADMIN_AUTH_TOKEN to inject a custom admin token and avoid the auth process.
• Add SHOPIFY_CLI_STORE so all Admin requests use this store, the user must have access to this store.

How to test your changes?

Execute this command to get the list of themes of the given store.
SHOPIFY_CLI_ADMIN_AUTH_TOKEN={your_token} SHOPIFY_CLI_STORE={store-name}.myshopify.com bundle exec shopify theme pull

The provided {your_token} must grant access to {store-name}.

Post-release steps

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).

[gonzaloriestra]

👏