Add canonical standardwebhooks platform and STANDARD_WEBHOOKS_BASE + factory; remove Twilio/PagerDuty

[Prateek32177]

Motivation

• Multiple providers share the Standard Webhooks (Svix-style) signing format but use different header names, so add a single canonical platform that supports header aliases instead of adding per-vendor entries.

Description

• Add standardwebhooks to the WebhookPlatform union and WebhookPlatformKeys enum to represent the canonical Standard Webhooks style.
• Introduce and export STANDARD_WEBHOOKS_BASE and createStandardWebhooksConfig(...) to provide a reusable base config and a factory for building Standard Webhooks signature configs with header aliases.
• Add a single standardwebhooks entry to platformAlgorithmConfigs using createStandardWebhooksConfig(...) with webhook-* headers and svix-* aliases, leaving other platform shapes intact.
• Update the algorithm verifier to honor signatureHeaderAliases and timestampHeaderAliases when extracting signatures and building the signed payload; remove Twilio/PagerDuty-specific platform entries and the Twilio URL override plumbing from adapters and verifier messaging.
• Export the new helpers from the package root and update documentation to surface the canonical Standard Webhooks usage and examples.

Testing

• Ran the test suite with npm test (executes ts-node src/test.ts) and all tests passed successfully.
• Built the package with npm run build (tsc) and the TypeScript build completed without errors.

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