Add a simple helm chart for backend deployment only + publish pipeline

[spwoodcock]

• As promised, a helm chart for deploying the fAIr backend only 😄
• It doesn't include a frontend, as that's handled by Github CI/CD --> S3 + Cloudfront.
• Also doesn't include a bundled database, as it relies on an external one (CNPG).
• Obviously this doesn't work too well for self-hosters, but will anyone be self hosting our new v3 infra anyway?

Justfile inclusion

• I copied my standard chart justfile I'm using elsewhere for building and publishing the helm chart.
• Adding in the main Justfile means we can add local dev commands or CI/CD commands as Just recipes now (a better alternative to Makefile) - a few other repos use this already.