Support for HTTP certificate URLs

I'm trying to get a project working with [LocalStack](https://www.localstack.cloud/), and `sns-validator` is currently rejecting all messages from the LocalStack SNS emulator because the certificate URLs all use `http://` instead of `https://`. It would be nice if the `MessageValidator` could be configured to check for `http://` domains, similarly to how it supports checking for a custom host.

Start at the MessageValidator implementation and trace how its custom-host configuration is defined and how certificate URLs are validated. Add equivalent configuration for permitted http:// certificate domains, then add or update validation tests to confirm LocalStack-style URLs work without weakening unrelated URL checks.

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