feat: Add header support to partywhen webhooks

[Ehesp]

This PR:

1. Fixes an issue with the readme docs not specifying an ID is required when creating a task
2. Adds support for passing headers to the webhook callback type, useful for when you need to pass things like signed JWTs so the webhook receiving the event can validate it's legit

[changeset-bot]

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Latest commit: 25fd1d2

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