Extract Payment Gateway Adapter plugin — move gateway drivers from core to plugin

[DawoudIO]

Goal: Extract payment gateway driver implementations and credential/config handling from core into a Payment Gateway Adapter plugin. Scope: Identify gateway drivers in finance/payment code, add plugin manifest and move provider code under src/plugins/core/payment-adapters/, update SystemConfig keys to plugin-prefixed keys and migration path. Acceptance: Core no longer contains gateway-specific implementations; plugin provides drivers and docs.

[github-actions]

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