sg: create a test command to run e2e tests locally

Created by: jhchabran

Add sg start enterprise-e2e to launch the local server with settings enabling to run the e2e test suite against it locally. sg test frontend-e2e has been patched to make it work.

I had to swap the order in which the envs are given to makeEnv to allow a command set to override environment variables from the underlying run targets, which is from the user perspective the expected behavior.

I added along the way support to fetch secrets from GCP and inject them in an env var. Those vars can still be overridden like the others, through GH_TOKEN=foobar sg ...

Also added a preamble key to commands, so we can display some content to the user before running a command.

Test plan

Got a green e2e test run locally.