Add Svix, Linear, PagerDuty and Twilio platform support; proxy URL handling and verifier enhancements

[Prateek32177]

Motivation

• Expand supported platforms to include svix, linear, pagerduty, and twilio and handle edge cases for proxied runtimes and Twilio URL canonicalization.

Description

• Added new platform algorithm configs and types for svix, linear, pagerduty, and twilio, updating platformAlgorithmConfigs, WebhookPlatform, and WebhookPlatformKeys.
• Enhanced request reconstruction to respect x-forwarded-proto and x-forwarded-host in toWebRequest so runtime requests behind proxies/CDNs use the public URL.
• Improved the algorithm-based verifier to support header aliases, comma/prefixed signature variants, Twilio URL overrides (twilioBaseUrl), Linear replay-window checks, Twilio bodySHA256 validation, and Svix id/timestamp aliases.
• Updated README to list new platforms and add a Twilio troubleshooting note, and extended src/test.ts with helper signature creators and tests for PagerDuty, Linear, Svix, and Twilio.

Testing

• Ran the verification test harness via node src/test.ts which includes the full webhook verifier smoke tests and the newly added PagerDuty/Linear/Svix/Twilio scenarios, and all checks passed.
• Existing platform verification scenarios (Stripe, GitHub, Clerk, etc.) were exercised by the same test run and continued to pass.
• No automated test failures were observed after these changes when running the repository test script (npm test).

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Codex Task