fix(vercel): Adds signature check to vercel delete endpoint

[Christinarlong]

Currently we're not doing a signature check on requests on the delete endpoint for vercel

Fixes VULN-1153

[linear-code]

VULN-1153 Unauthenticated Vercel uninstall webhook (DELETE) lacks signature verification

[michelletran-sentry]

Since all the endpoints in VercelWebhookEndpoint require signature validation, thoughts on creating a custom authentication class for this REST resource?

Maybe something like:

class VercelSignatureAuthentication(authentication.BaseAuthentication):
    def authenticate(self, request: Request) -> tuple[Any, Any]:
        if not verify_signature(request):
            raise AuthenticationFailed("Invalid signature.")
        return None

[github-actions]

Backend Test Failures

Failures on f1b800e in this run:

tests/sentry/integrations/vercel/test_webhook.py::SignatureVercelTest::test_get — log

tests/sentry/integrations/vercel/test_webhook.py:30: in test_get
    assert response.status_code == 405
E   assert 401 == 405
E    +  where 401 = <Response status_code=401, "application/json">.status_code

[michelletran-sentry]

LGTM!