[Feature] Replace all custom env. var. expansion logic for configuration files with `a8m/envsubst`

Description

graph-node supports env. var. expansion in certain sections of the configuration files, and we also ship envsubst in the official Docker image for more complex use cases. I'd argue it makes most sense to drop our custom env. var. expansion logic and automatically invoke envsubst in the start script. This benefits us in three ways:

1. Less code – yay, always a win.
2. We enable env. var. expansion in all sections of the configuration files, as opposed to just some of them.
3. We get support for default values and other advanced features of envsubst, like custom evaluation expressions.

Are you aware of any blockers that must be resolved before implementing this feature? If so, which? Link to any relevant GitHub issues.

No response

Some information to help us out

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