Encrypt webhook tokens and URLs in the database

What does this MR do?

Introduces the following database columns

• web_hooks.encrypted_token
• web_hooks.encrypted_token_iv
• web_hooks.encrypted_url
• web_hooks.encrypted_url_iv

These shadow the web_hooks.token and web_hooks.url columns.

attr_encrypted is used to store new webhook tokens and URLs encrypted in the database.

Unencrypted token and url values whenever a webhook updates.

I still need to figure out how to migrate the existing data proactively in a background migration.

I think we'll need an %11.5 follow-up to remove the fall-back-to-unencrypted-fields logic, and the unencrypted columns itself. Need to investigate that in some more detail.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #51021