Add webhook signature verification helper #3

The SDK lets you register a webhook_url when creating an experiment
(lab.create_experiment(..., webhook_url=...), client.experiments.create(..., webhook_url=...)),
but it has no support for the other half of that flow: verifying the deliveries that arrive. The
only path the SDK offers for getting results is polling with client.experiments.get_results(...).

So today every integrator hand-writes the HMAC check from the snippet in the API introduction docs.
The snippet itself is correct. The code around it is where things go wrong, and the failure modes
are quiet ones:

• Re-serializing the parsed body (json.dumps(request.json())) before signing produces different
  bytes than the ones that were signed, so every signature fails. The usual "fix" is to switch
  verification off.
• Comparing hex digests with == instead of hmac.compare_digest.
• An unset secret. hmac.new(b"", body, sha256) returns a perfectly valid signature, so a missing
  WEBHOOK_SECRET makes verification pass rather than fail. The control disarms itself and still
  looks fine from the outside.
• A missing or malformed X-Adaptyv-Signature raising TypeError or ValueError out of the
  handler instead of being refused as an untrusted request.
• Deliveries retry up to 3 times, so a handler that 500s once is guaranteed a duplicate, and
  X-Adaptyv-Delivery-Id is the only thing that lets you dedupe.

Proposal: a small framework-agnostic module, adaptyv/webhooks.py, with one entry point.

from adaptyv.webhooks import verify

event = verify(body=raw_bytes, headers=request.headers, secret=SECRET)
event.event         # "experiment_update"
event.delivery_id   # "019b8da3-4a91-16c6-fa94-619212bee6a6"
event.payload       # the parsed body

• Takes the raw request body as bytes. A str or a dict is refused, so the re-serialization
  mistake is not expressible through the API.
• Raises rather than returning a boolean, because a boolean is what gets assigned to a variable
  and then never checked. Two new leaves under the existing AdaptyvError, kept as siblings so
  that catching one cannot swallow the other: WebhookVerificationError means the delivery never
  proved it came from Foundry and should be answered 4xx, WebhookPayloadError means it did and
  the envelope was unreadable. That one must not be answered 4xx, since 4xx is permanent in the
  retry model and would discard a real event.
• Stdlib only (hmac, hashlib, json). No new runtime dependencies and no framework imports.
• Tests need no API key and no network. Signatures are constructed in the test with the same
  secret, so the suite is deterministic for anyone who clones the repo.

Plus a short README section: a framework-agnostic example, a FastAPI example, and a note that
handlers have to be idempotent because of the retry behavior.

A PR implementing this is open against main. Happy to change the shape of it, or to close it,
if you would rather take a different approach.