Add gRPC Auth

[amouat]

Investigate adding encryption and auth to front end/back end communication (see architecture diagram). This probably means mutual TLS auth.

In the short term this isn't a large issue as the front-end and back-end are in the same pod and the back-end isn't exposed outside of the pod. But as we look at becoming distributed and H/A this will become important.

[blaggacao]

Absolutely in line with #184

[amouat]

This is slightly different - it's about interior communication between Rocket and the "backend", which occurs via GRPC. The idea is to allow distributed architectures in the future, but at the moment it's an overhead.

[blaggacao]

SPIFFE could (as one option) provide the mTLS machinery that might eventually be required.