feat: allow for ingress.host templating

[tumido]

Description of the change

In some cases it may be useful to allow ingress.host to be templated - for example if you want to reuse the same value in backstage.appConfig. In scenarios where Backstage chart is used as a subchart the only way to properly propagate templated values is through global scope (you can't reference ingress.host outside of the subchart). This PR should add support for such scenarios:

# values.yaml
global:
  foo: foo.bar
ingress:
  host: '{{ .Values.global.foo }}'
  enabled: true

Existing or Associated Issue(s)

Additional Information

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the values.yaml and added to the README.md. The helm-docs utility can be used to generate the necessary content. Use helm-docs --dry-run to preview the content.
• JSON Schema generated.
• List tests pass for Chart using the Chart Testing tool and the ct lint command.