Add compliance portal visitor OAuth service

Handle OAuth initiation, callback exchange, and session creation
for anonymous trust center visitors through the portal package.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/complianceportal/visitor/errors.go

@@ -23,6 +23,8 @@ package visitor
 import "errors"
 
 var (
+	ErrOAuthStateNotFound        = errors.New("oauth state not found")
+	ErrOAuthStateExpired         = errors.New("oauth state expired")
 	ErrPageNotFound              = errors.New("page not found")
 	ErrMembershipNotFound        = errors.New("membership not found")
 	ErrUserNotFound              = errors.New("user not found")