Unify configuration in Stargate v2 services

[olim7t]

Currently we have a mix of different approaches:

• command-line arguments managed with the "airline" library, for example in RestServiceStarter
• Dropwizard options, for example sgv2-restapi/src/main/resources/config.yaml
• system properties loaded at random places in the code, for example BridgeConfig.ADMIN_TOKEN (this one is mine, but there are others in the v1 code)

We should centralize options in one place. I think command-line arguments is the best place, as they are documented and easy to find.