Allow env var configuration of updated transformation config

[colmsnowplow]

In #169 we instrument a new, more sensible way of configuring transformations.

This change replaces our old hacky workaround transformation configuration with HCL - which supports arrays and allows us to be much more declarative in our config.

However, the downside is that it's not configurable via env var. This task is to introduce an env var which allows us to provide a base-64 encoded hcl configuration, which we will parse and use as the transformation config.