Refactor the GDK ERB Template logic

What does this merge request do and why?

This improves on how the ERB templates are handled:

• Refactor the GDK::ErbRenderer into GDK::Templates module
• Small improvements on the ErbRenderer logic
• Documentation
• Added a template Context class, to hold general helper methods
• No longer required to declare ALL services

All changes should be backward compatible with existing templates

How to set up and validate locally

Automated tests and integration tests should cover the use-cases

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.