Add more options to run custom scripts at multiple points in the build stages

[lreiher]

• introduces new configuration variables to specify custom scripts that run at different points in the build process
  • BEFORE_DEPENDENCY_IDENTIFICATION_SCRIPT: runs in dependencies stage after basic setup, before .repos are cloned
  • BEFORE_DEPENDENCY_INSTALLATION_SCRIPT: runs in dependencies-install stage as first step in .install-dependencies.sh
  • AFTER_DEPENDENCY_INSTALLATION_SCRIPT: runs in dependencies-install stage as last step in .install-dependencies.sh
• CUSTOM_SCRIPT_FILE effectively has been renamed to AFTER_DEPENDENCY_INSTALLATION_SCRIPT, but still defaults to docker/custom.sh
• this feature should add a lot more customizability of the build process, useful/required for, e.g., setting up private apt repositories (closes Is it possible to add a private debian repository? #23, closes Adding Rosdep Sources Before Rosdep Install #49)

[lreiher]

To be tested with arm64/ros-monitoring by @lukaszanger before merging.

The merge-base changed after approval.