Custom environment metadata in JSON artifacts

[beckjake]

Describe the feature

Include custom environment-sourced metadata in the JSON artifacts.

Inside the metadata field (or at the root if #2761 is not done yet), dbt should add a field - tentatively env. This field should be a dictionary, populated from the contents of dbt's environment that start with DBT_ENV_CUSTOM_ENV_. The keys should be the environment variables without the prefix, and the values the values. For example:
DBT_ENV_CUSTOM_ENV_run_id=1 DBT_ENV_CUSTOM_ENV_run_env=ci dbt run
will result in a manifest with {"env": {"run_id": "1", "run_env": "ci"}} in its metadata field.

See this comment for a tiny bit of context.

Split off from #2671