Add generation for Extension example

[timuthy]

How to categorize this PR?

/area ops-productivity
/kind enhancement

What this PR does / why we need it:
This PR adds the generation for an Extension example manifest, similar to example/controller-registration.yaml which will be replaced on the long run.

Release note:

An example `Extension` manifest for extension registration has been added. It can be found at [`example/extension.yaml`](https://github.com/gardener/gardener-extension-shoot-oidc-service/blob/master/example/extension.yaml)

[dimityrmirchev]

/lgtm

[timuthy]

/area ipcei