Loading partial pipeline configuration from the environment

Is your feature request related to a problem? Please describe.
As a user of data prepper, I would like the ability to load part of my configuration from the environment. This would allow me to deploy the same pipeline configuration file into different environments and limit the number of pipeline configuration files I need to manage. My core business logic would stay the same while my environments change.

Describe the solution you'd like
The pipeline configuration files would support reading environment variables into the pipeline.

Example config snippet where the host url is loaded by and environment variable OPENSEARCH_URL:

sink:
   - opensearch:
     hosts:
       - ${OPENSEARCH_URL}

No file or test is named in the issue. Start by tracing how Data Prepper loads pipeline configuration, then identify where environment-variable substitution would fit; done means a pipeline configuration can resolve values such as ${OPENSEARCH_URL} while preserving the shared configuration across environments.