sdk: add a webhook signature verification helper

[frankie567]

Similar to the one we added manually in Speakeasy SDK:

• https://github.com/polarsource/polar-python/blob/f069b317d1402eb8db3bf8c8065ba58f8ddc0b54/src/polar_sdk/_webhooks/__init__.py#L119-L135
• https://github.com/polarsource/polar-js/blob/63510677499a41e9694be83a96aafbafc750d7f6/src/webhooks.ts#L135-L151

They're both using the standardwebhooks dependency. It should be added directly in their respective webhooks.{py|ts} module below the payload definitions.