Loader: make it possible to use environment variables for Iglu resolver

[benjben]

The json for Iglu resolver needs to be completely "rendered" when the loader starts, it can't contain any environment variable (e.g API_KEY), which is an issue when defining a task definition for loader in ECS and storing secrets in SSM.

2 possible ways to solve this:

1. make it possible to define Iglu resolver as an environment variable, so that the whole json with the secrets can be stored in SSM
2. make it possible to specify environment variables that get replaced in the resolver config

[piankris]

Has it been resolved yet?