Build admin freeze/unfreeze invoice helper

[Kingsman-99]

Labels: complexity: high

Add SDK methods wrapping the contract's admin_freeze and admin_unfreeze functions. These are admin-only operations and the SDK should enforce that only an authorized admin keypair can call them.

Acceptance criteria:

• adminFreezeInvoice(invoiceId, reason, adminKeypair) submits freeze transaction
• adminUnfreezeInvoice(invoiceId, adminKeypair) submits unfreeze transaction
• Both methods verify the caller is an admin before building the transaction
• Emits correct audit event after submission

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[edrizxabdulganiyu-blip]

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