P3-1: Restructure payment-adapter-webhook with new packages

[tcheeric]

Description:
Restructure webhook module packages into core, spi, and exception.

Changes required:

• Create core, spi, and exception packages under webhook.
• Move existing classes into the new package structure.

Acceptance criteria:

• Webhook module package structure matches the design.
• Classes compile with updated package locations.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-3-task-1
Dependencies: P2-5