refactor: validate issuer (url) when retrieving configuration

Closes #26

Author: ColinFrick

Sources/FusionAuth/oauth/OAuthAuthorizationService.swift

@@ -90,6 +90,12 @@ extension OAuthAuthorizationService {
                     return
                 }
 
+                let scheme = config.issuer?.scheme
+                if scheme != "http" && scheme != "https" {
+                    continuation.resume(throwing: OAuthError.invalidIssuer)
+                    return
+                }
+
                 continuation.resume(returning: config)
             }
         }

Sources/FusionAuth/oauth/OAuthErrors.swift

@@ -9,4 +9,5 @@ enum OAuthError: Error {
     case refreshTokenNoResponse
     case unableToUpdateInternalState
     case invalidUserAgent
+    case invalidIssuer
 }