[K8s Operator] UI Deployment branding with extended CRD

[CarlosLanderas]

What this PR does / why we need it:

This PR extends the HealthChecks UI CRD and Operator to allow the user configure branding in a very simple way.

The user just needs to configure stylesheetPath and styleSheetContent with custom css in the yaml definition and the operator will configure all the UI docker image necessary environment variables and create a new ConfigMap with the css content stored as binary data. This ConfigMap will be configured as a volume source so the UI application can read it's contents and server the stylesheet to the frontend.

Sample to easily deploy a HealthChecks UI environment with World of Warcraft styling:

apiVersion: "aspnetcore.ui/v1"
kind: HealthCheck
metadata:
  name: healthchecks-ui
  namespace: lande
spec:
  name: healthchecks-ui
  servicesLabel: HealthChecks
  serviceType: LoadBalancer
  stylesheetPath: css/dotnet
  stylesheetContent: >
    :root {    
      --primaryColor: #2a3950;
      --secondaryColor: #f4f4f4;  
      --bgMenuActive: #e1b015;
      --bgButton: #e1b015;
      --logoImageUrl: url('https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/WoW_icon.svg/1200px-WoW_icon.svg.png');
      --bgAside: var(--primaryColor);   
    }

Screenshot

Does this PR introduce a user-facing change?: No