Add Backstage charts to BB umbrella repository

Base requirements

The following files need to be added to bigbang/chart/templates/backstage

• git-credentials.yaml
• gitrepository.yaml
• helmrelease.yaml
• imagepullsecret.yaml
• namespace.yaml
• values.yaml

Additionally an entry within bigbang/chart/values.yaml under addons:

Additional

If any other backstage-related templates need adding to the umbrella chart please note them here.