Provision DomainMapping resource for created Knative Service from GithubSource and use DomainMapping URL as the Webhook URL

[dreamrdeceiver]

Problem
When the GithubSource CR is applied the controller creates a Knative Service, exposes the Service URL following the convention of name.namespace.custom-domain.com and adds it as a GitHub webhook on the repository.

This is a problem in some managed clusters (like SAP Kyma) as Let's Encrypt certificates are pre-provisioned and is only valid for *.<identifier>.kyma.ondemand.com

Due to organization restriction policies this cannot be modified from the platform provisioning team.

When this URL is added as a GitHub webhook tls verification fails because of -

tls: failed to verify certificate: x509: 
certificate is valid for *.<identifier>.kyma.ondemand.com, not for 
<name>.<namespace>.<identifier>.kyma.ondemand.com

I think we can overcome this restriction by using Knative DomainMapping resource to map it to a shorter domain and set that as the webhook URL.

I think this can be solved by adding an optional spec for example -

apiVersion: sources.knative.dev/v1alpha1
kind: GitHubSource
metadata:
  name: my-githubsource-sample
spec:
  githubAPIURL: "https://github.tools.sap/api/v3/"
  eventTypes:
    - pull_request
  ownerAndRepository: "myOrg/sample"
  domainMapping: # Optional
    name: github-<unique-identifier>.custom-domain.com
    tls: # Optional
      name: <tls-secret-name>
   ....
  sink:
    ref:
      ...

which would result in the following DomainMapping -

apiVersion: serving.knative.dev/v1alpha1
kind: DomainMapping
metadata:
  name: github-<unique-identifier>.custom-domain.com
spec:
  ref:
    name: my-githubsource-sample-n5lsc # <- generated name suffix
    kind: Service
    apiVersion: serving.knative.dev/v1
# tls block specifies the secret to be used
  tls:
    secretName: <tls-secret-name>

Then use the URL from DomainMapping status and set that as a GitHub Webhook

This would be very helpful for developers in organizations to not do any long ticketing Ops to get custom domain / DNS setup.

Additional Context

The need for DomainMapping is because this resource cannot be applied to cluster in a GitOps approach because of generated name being used for the Knative Service name and also any manual updates made to the webhook URL will be replaced when the controller reconciles.

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