Picks, from the methods a client declares, the one this request presented, falling back to the first declared.
the client's configuration (clientConfigOf())
clientConfigOf()
request, clientSecret (the presented secret), assertion and assertionType
request
clientSecret
assertion
assertionType
the method to verify, or '' where none is declared
Picks, from the methods a client declares, the one this request presented, falling back to the first declared.