Add from_env_var HCL function

[traut]

Background

Hard-coding credentials in .fabric files is a bad practice, so there should be a way to load credentials during execution dynamically.

Design

Provide HCL function from_env_variable() that will take the name of the environment variable and load the value from it during execution (if the variable is set)

Important

Related issue: #4

Usage example:

// plugin configuration
config data elasticsearch "clusterB" {
    cloud_id = from_env_var("CLUSTER_B_CLOUD_ID")
    api_key = from_env_var("CLUSTER_B_API_KEY")
}

or for the inline config block

data elasticsearch "critical_alerts" {
  config {
    cloud_id = from_env_var("CLUSTER_B_CLOUD_ID")
    api_key  = from_env_var("CLUSTER_B_API_KEY")
  }

  index        = ".alerts-security.alerts-*"
  query_string = "kibana.alert.severity:critical AND @timestamp:[now-1d/d TO now]"
  size         = 10
}

Deliverables

• from_env_var() function, to be executed during parsing of the templates
• unit tests for the function