Make githubsource's SecretToken optional

[lionelvillard]

From knative/eventing-contrib#297

Right now SecretToken appears to be required:
https://github.com/knative/eventing-sources/blob/master/pkg/apis/sources/v1alpha1/githubsource_types.go#L66

I think it would be nice if it were made optional, and if not provided then the system just created something to use (like a UUID). This value has no meaning outside of this subscription so I don't need the requirement to force the user to specify it. However, if someone does want to do so they can.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[lionelvillard]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[lionelvillard]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[lionelvillard]

/remove-lifecycle stale