Update DWP CASA example with DWP Frontend

[htmlandbacon]

What

We should update https://github.com/dwp/govuk-casa with a configured example with our frontend package

Why

We already have internal examples of express projects in Health - but it would be good to have a CASA example, as this will help people better understand how the parts are released.

Done when

• [x] Integrated example using header/keybar/footer
• [x] Small journey of a agent service
• [x] Custom css and js build
• [x] ~Review if we should have a template (based on internal casa ticket)~ - decided not useful at this time.

Outcomes

Example version within the CASA folder, some understanding of how we link to resources.

[htmlandbacon]

Out for review with casa maintainers

[htmlandbacon]

This is now in the casa examples folder