feat: Add Digital Signature and Validate Signature tools

New Features:
- Digital Signature tool: Sign PDFs with X.509 certificates (PFX/PEM)
  - Visible and invisible signatures
  - All pages, first page, last page, or custom page selection
  - Dynamic signature height based on text content
  - Custom signature text, image, and styling options

- Validate Signature tool: Verify digital signatures in PDFs
  - Extract and parse PKCS#7 signatures
  - View signer and issuer certificate details
  - Check certificate validity and expiry
  - Optional custom X.509 certificate for trust verification
  - Full/partial coverage detection

Infrastructure:
- Added Cloudflare Worker CORS proxy for certificate chain fetching
- Updated README with new tools and proxy deployment instructions

Documentation:
- Added Digital Signature and Validate Signature to README tools table
- Added CORS proxy deployment guide for self-hosters

cloudflare/wrangler.toml

@@ -0,0 +1,16 @@
+name = "bentopdf-cors-proxy"
+main = "cors-proxy-worker.js"
+compatibility_date = "2024-01-01"
+
+# Deploy to Cloudflare's global network
+# If you are self hosting change the name to your worker name
+# Run: npx wrangler deploy
+
+# Optional: Custom domain routing
+# routes = [
+#   { pattern = "cors-proxy.bentopdf.com/*", zone_name = "bentopdf.com" }
+# ]
+
+# Optional: Environment variables
+# [vars]
+# ALLOWED_ORIGINS = "https://www.bentopdf.com,https://bentopdf.com"