[CT-1990] Parse access property on model node

[MichelleArk]

Requirements

• access is a new property on model nodes. For phase 1, cannot be provided for any other 'groupable' nodes
• access property should be defined as an enum, for potential extension in the future. Currently must be one of private or public, or protected:

• private - ref works within the same group
• protected - ref works within the same project/package
• public - ref works across projects/packages

• access defaults to protected, irrespective of the group config value

Reference spec

# models/marts/github/github.yml

groups:
  - name: github
    owner: 
      name: Jeremy
      email: data@jer.co

models:
  - name: int__github_issue_label_history
    access: private
    config: 
      group: github  # explicit opt-in
  - name: fct_github_issues
    access: protected
    config: 
      group: github  # explicit opt-in
  - name: fct_github_issues
    access: public