Provide bootstrap.yaml as an alternative to environment variables

[pront]

The https://vector.dev/docs/reference/cli/#environment-variables list is growing larger. Instead of relying on environment variables, we can have a YAML file to configure all these parameters in one place.

The DD agent resolution process seems like a great fit for Vector as well:
https://docs.datadoghq.com/agent/configuration/proxy/?tab=linux#environment-variables