What does this PR do

• Remove the syntax necessary to work with environment variables in custom properties;
• Add optional property env to CustomProperty type, whose default value is set to false;
• Internally, decode optional Bool to non-optional isEnvironmentVariable.

How can it be tested

• Adhere to the latest changes in variants.yml
• Include a custom property whose value reads from environment variable:

 - name: sample_property
    value: NAME_OF_ENV_VAR
    env: true
    destination: project

• Switch to this respective variant variants switch --variant A_VARIANT
• IF iOS Variants.swift should contain struct Secrets with your encrypted static var.
• IF Android gradleScripts/variants.gradle should contain:

System.getenv('NAME_OF_ENV_VAR')

Task

resolves #121

Checklist:

• I ran make validation locally with success
• I have not introduced new bugs
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors