Missing conformance checks and e2e test

[cowbon]

Several checks are still missing from ratsd

1. Nonce validation. There should be a data size limit for the nonce
2. Ratsd should issue bearer tokens for request authentication
3. The E2E test and smoke tests for every builds

[thomas-fossati]

With regards to

The E2E test and smoke tests for every builds

I think unit tests may be sufficient.

Besides, we'll need to set a coverage threshold and check that a PR doesn't decrease it.