feat: implement REST API views and HMAC-based webhook authentication for event ingestion

[Dantama022]

Webhook HMAC Validation Decorator Implementation

I have implemented a new HMAC validation decorator for SoroScan webhooks. This allows receiver endpoints to verify the authenticity of incoming webhooks from SoroScan.

Changes Made

Decorator Implementation I created decorators.py which contains the webhook_hmac_required decorator.

Extracts contract_id from the JSON payload.
Retrieves the applicable WebhookSubscription secret from the database.
Verifies the X-SoroScan-Signature header (supporting SHA-256 and SHA-1).
Uses hmac.compare_digest to prevent timing attacks.
Sample Endpoint
I added a sample receiver endpoint in views.py and registered it in urls.py.

Endpoint:
POST /api/ingest/webhooks/receiver-example/

Protected by @webhook_hmac_required()
Tests
I created
test_decorators.py
with comprehensive tests for:

Valid SHA-256 and SHA-1 signatures.
Invalid and missing signatures.
Missing contract ID and inactive subscriptions.
Verification Results
Linting and Formatting
Ruff: All checks passed.
Formatting: Applied ruff format to all new and modified files.

Closes #334

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