Add Svix, Linear, PagerDuty and Twilio webhook platform support and verification

[Prateek32177]

Motivation

• Add support for four additional webhook providers (Svix, Linear, PagerDuty, Twilio) and improve signature/header parsing to cover provider variations and edge cases.
• Provide provider-specific protections such as Linear replay-window checks and Twilio payload hash validation.
• Make detection and configuration more robust by allowing header aliases and alternative signature formats.

Description

• Added new platform entries to src/types.ts and added rows to README.md to document svix, linear, pagerduty, and twilio (marked as untested in README).
• Introduced platform configurations for svix, linear, pagerduty, and twilio in src/platforms/algorithms.ts including header names, algorithms, and custom config flags (e.g. idHeaderAliases, timestampHeaderAliases, comparePrefixed, validateBodySHA256).
• Extended platform detection in src/index.ts to detect the new headers and disambiguate svix vs clerk when svix-signature is present with an svix-id.
• Enhanced signature parsing and payload handling in src/verifiers/algorithms.ts to support comma-separated and prefixed variants (including v1, and v1= forms), header alias lookup for timestamps/ids, custom payload {url} formatting for Twilio, and optional comparePrefixed logic used for PagerDuty.
• Implemented provider-specific verification helpers in src/verifiers/algorithms.ts including validateLinearReplayWindow for Linear timestamp replay protection and validateTwilioBodyHash for Twilio bodySHA256 validation, and integrated these checks into the verification flow.
• Added helper signature generators and end-to-end tests for the new platforms in src/test.ts (createPagerDutySignature, createLinearSignature, createSvixSignature, createTwilioSignature) and appended tests (26–29) exercising PagerDuty, Linear, Svix, and Twilio verification paths.

Testing

• Executed the automated test runner implemented in src/test.ts which includes the new tests for pagerduty, linear, svix, and twilio, and the new checks completed successfully.
• Existing platform verification tests were re-run as part of the same suite and also passed.

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Codex Task