Add fetch strategy support templates

Adds reusable templates for HAF apps that want to use GIT_STRATEGY=fetch
instead of clone. Fetch strategy reduces GitLab server load by reusing
workspace between jobs.

New templates:
- .haf_fetch_strategy_variables: Variable definitions to copy
- .haf_git_corruption_cleanup: Pre-get-sources hook for cleaning corrupt state
- .haf_submodule_init: Manual HAF submodule initialization
- .haf_fetch_strategy_example: Shows all pieces together

Usage:
  default:
    hooks: !reference [.haf_git_corruption_cleanup, hooks]
  my_job:
    before_script:
      - !reference [.haf_submodule_init, script]