Enable placeholders on configuration files

Feature description

Ability to declare placeholder values in our configuration files that will be taken from the environment variables.

Use Case

Sometimes we do not decide how our environment variables will be named, so we don't get to define them with prefixes like APP__

It would be nice if we could declare placeholders in our configuration files where the values should be taken from our environment variables.

Example:

application_port: ${PORT} #value taken from PORT environment variable
database:
  host: "127.0.0.1"
  port: 5432
  username: "postgres"
  password: "${DB_PASS}" #value taken from DB_PASS environment variable
  database_name: "${DB_HOST}" #value taken from DB_HOST environment variable
  require_ssl: false

The issue names no files, tests, or entry points. Start by locating the configuration-file parsing and environment-variable handling, then determine how placeholders such as ${PORT} should be interpreted across supported formats. Done means configuration values can be sourced from the referenced environment variables, including unprefixed names such as PORT and DB_PASS, with tests covering the requested YAML example.