tkn pac setup webhook: let the user specify a webhook secret

[chmouel]

We generate a secret automatically, but if the user didn't create a repo she/he would not know which secret that has been setup on the github interface (it's hidden). Let allow specifying a webhook secret via a flag :

--webhok-secret

[ramessesii2]

/assign

[ramessesii2]

@chmouel I'd like to raise a PR for it! Can you assign it to me? 😺