Make githubsource's SecretToken optional

From https://github.com/knative/eventing-contrib/issues/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.

Start with pkg/apis/sources/v1alpha1/githubsource_types.go at the referenced SecretToken field, then trace how that value is consumed when a GitHubSource is created. Confirm the expected behavior for an omitted token and update the relevant implementation and validation coverage so both omitted and explicitly supplied tokens work.

Written by the indexing model from the issue text.