Add prepend_mod in Gitlab::SubscriptionPortal::Client for JH

What does this MR do and why?

Custom Dot inJH provide api "manage/api/v1/subscriptions.json?plan_code=#{plan_code}" to fetch all custom purchase list, so JH team need reuse subscription portal client, so we add prepend_mod in Gitlab::SubscriptionPortal::Client, related MR https://jihulab.com/gitlab-cn/gitlab/-/merge_requests/2123

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@daveliu @tianlu1677