Webhook encrypted tokens: Backend support

This is part of the epic &7970 (closed). See that epic for more context.

Proposal

A separate issue captures the database work #361123 (closed).

The steps in this issue are:

• backend Support for saving encrypted tokens from the frontend.
• Tokens should be substituted into the URI when the webhook executes.

Availability & Testing

Suggestion: Add a request spec around saving encrypted key/value token pairs with a webhook, and that the webhook URI secrets are encrypted on fetch. package-and-qa should also be run on the MR.

It could be good to follow up with an feature/e2e spec with the frontend work. We have e2e tests around webhooks that should be expanded.