OAuth: Fix token refresh failure when custom SSL settings are configured for OAuth provider (#27523)

OAuth token refresh fails when custom SSL settings are configured for 
oauth provider. These changes makes sure that custom SSL settings 
are applied for HTTP client before refreshing token.

Fixes #27514

pkg/api/pluginproxy/ds_proxy_test.go

@@ -383,6 +383,9 @@ func TestDSRouteRule(t *testing.T) {
 					Config: &oauth2.Config{},
 				},
 			}
+			setting.OAuthService = &setting.OAuther{}
+			setting.OAuthService.OAuthInfos = make(map[string]*setting.OAuthInfo)
+			setting.OAuthService.OAuthInfos["generic_oauth"] = &setting.OAuthInfo{}
 
 			bus.AddHandler("test", func(query *models.GetAuthInfoQuery) error {
 				query.Result = &models.UserAuth{