feat: add locale override parameter to client.fetch

[matthewvolk]

What/Why?

Allows passing a locale that will be used to set the outgoing "Accept-Language" header sent to the BigCommerce Storefront GraphQL API. This additional, optional parameter gives you more explicit control over what language you'd like responses to be translated into, leveraging BigCommerce's resolved locale feature.

Note

This PR originated from #2286

Testing

N/A - Additive change

Migration

• For those consuming the package from the NPM registry, this feature will be available in version 0.16 or higher, so a simple version bump should suffice.
• For those using the package via pnpm workspaces, simply cherry-pick the additive diff from this PR into your branch.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9e1ad83

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@bigcommerce/catalyst-client	Minor
@bigcommerce/catalyst-core	Patch

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[matthewvolk]

Converted to draft; discussed with the team, and we should either more appropriately name this parameter based on what it is actually doing (overriding Accept-Language header), or find a way to deliver #2286 without a change to the client. Going to think a bit on the best path forward, but don't want to give the impression this PR is final.

[matthewvolk]

I'm going to go the route of not modifying the client. Feels aggressive to extend the client's API just for the one case needed in #2286