Expose env vars as FCL variables

[traut]

Background

The function from_env_variable() we support currently allows template authors to load dynamic values from environment variables. While it is very useful, it lacks the flexibility we can achieve with FCL variables

Design

Expose env variables under env. namespace in FCL context, so it can be used with HCL attributes / templating

For example:

config data somesource {
  uril = "https://${env.SOMESOURCE_DOMAIN}/api"
  username = env.SOMESOURCE_USERNAME
  password = env.SOMESOURCE_PASSWORD
}

Also, from_env_variable() function should be retired