GitHub OCI requires implementation for fulcio CA

[lukehinds]

PR #180 missed the modular CA implementation and does not work for fulcioca.

from a cursory look we need a challenges.GithubWorkflowValue to manage subject.Value

fulcio/pkg/ca/fulcioca/fulcioca.go

Lines 45 to 54 in 922b7cd

	switch subject.TypeVal {
	case challenges.EmailValue:
	cert.EmailAddresses = []string{subject.Value}
	case challenges.SpiffeValue:
	challengeURL, err := url.Parse(subject.Value)
	if err != nil {
	return "", nil, err
	}
	cert.URIs = []*url.URL{challengeURL}
	}

cc @mattmoor