feat(webhook): allow webhook without secret

[KeXiangWang]

I hereby agree to the terms of the RisingWave Labs, Inc. Contributor License Agreement.

What's changed and what's your intention?

Currently, webhook strictly depends on the secret management feature, which is a premium feature now. However, as it's actually a common and lightweight feature, it's unreasonable to make webhook a premium feature as well. This PR allow user to use raw string as secret to do webhook verification. Example:

create table validate_raw_string (
  data JSONB
) WITH (
  connector = 'webhook',
) VALIDATE AS secure_compare(
  headers->>'x-hub-signature',
  'sha1=' || encode(hmac('TEST_WEBHOOK', data, 'sha1'), 'hex')
);

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note