config: Review config env var resolution to configmap

[MikeGoldsmith]

LGTM.

By the way, it might be worth looking at: https://github.com/open-telemetry/opentelemetry-collector/tree/main/confmap

I'm not sure if declarative config is thinking of adding custom resolvers, but currently the collector does inside-out resolution to allow for things like:

${secretsmanager:${env:NEWRELIC_OTLP_ENDPOINT}}

Originally posted by @herin049 in #5091 (comment)