feat(cli): Allow specifying options using env vars

[RomainMuller]

Allows passing options to the cdk command using environment variables
with the CDK_ name prefix. This can be used, for example, for the
following:

• CDK_VERSION_REPORTING=false disables version reporting
• CDK_REQUIRE_APPROVAL=broadening requires approval for changes to
  security policies that broaden the grants
• CDK_VERBOSE=true enables verbose logging in the toolkit

Added a validator in cdk doctor that will display the environment
variables that may be picked up by the toolkit, and highlight those that
are reserved for internal use by the toolkit (CDK_CONTEXT_JSON and
CDK_OUTDIR).

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Pull Request Checklist

• Testing
  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.