test: run the Tokenserver E2E tests without a cached JWK

[ethowitz]

Description

Runs the Tokenserver E2E integration tests against a Tokenserver without a cached FxA public JWK. This was done to ensure that we have test coverage for the OAuth verification code paths that involve making a request to FxA to verify tokens. We didn't have coverage before and it hid a bug where we were incorrectly handling BlockingErrors returned by Actix's web::block.

Testing

I made sure that the test_unauthorized_oauth_error_status test failed without ebdd609 and passed with it

[ethowitz]

@jrconlin Good call! I've updated the doc comment on Tokenserver's Settings struct. Also, I've added a few new tickets to make updates to syncstorage-rs's documentation:

• https://mozilla-hub.atlassian.net/browse/CONSVC-1425 (turn on the missing_docs lint, which will require any public item to have a doc comment)
• https://mozilla-hub.atlassian.net/browse/SYNC-3341 (auto build and deploy syncstorage-rs's cargo docs on CI, similar to what we have for Merino)
• https://mozilla-hub.atlassian.net/browse/SYNC-3342 (audit and rewrite README, add an ARCHITECTURE.md file)

[ethowitz]

Sorry for the noise, had to rebase -- I'll merge this right away after it's re-approved