auth: use a centrally stored jwt signing private key

[iulianbarbu]

Description of change

We want our auth service to not generate a new private key for JWT signing at each release. This change should help in not invalidating the cached JWTs on the gateway when auth is released in isolation.

• start auth service based on an environment secret passed through a flag, for JWT signing.
• local development will use a default secret set in the docker-compose.dev.yml, to not require setting it up in the environment before starting the stack locally (the secret is compromised since it is stored in version control and we shouldn't use it for anything outside local development).
• the JWT signing private key parsing logic will fail early in case the provided key is not valid (e.g when empty if forgetting to set it)

How has this been tested? (if applicable)

• checked the parsing locally by starting the auth service
• Tested on staging:

1. deployed all services
2. deployed a project
3. scaled auth service down to 0 and then up to 1
4. rerun immediately a command
5. checked auth logs and no JWT conversion happened in auth while doing 4 (meaning a cached JWT was used successfully).

[oddgrd]

Makes sense! I left one question.

[chesedo]

Really nice!! 🥳

Left small comments.

[chesedo]

Small nit