Support Account buildstorage

https://wiki.guildwars2.com/wiki/API:2/account/buildstorage

Use the API page linked in the issue body and inspect how this repo already implements other Account endpoints in Swift. Identify the matching request/route layer and tests for account methods, then add buildstorage there using the same pattern. First run the project’s existing tests (especially API/account-related ones) before and after changes. The issue is done when the wrapper exposes account buildstorage support and tests pass.

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