MM-10417 Improve HTTPService for use in image proxy (#9966)

* Replaced httpservice with proper http.Client

* Added HTTPService.MakeTransport

* Expose timeouts used by HTTPServiceImpl

* Add additional documentation to HTTPService

* Remove MockedHTTPService

* Fix missing license

utils/testutils/mocked_http_service.go

@@ -1,30 +0,0 @@
-// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
-// See License.txt for license information.
-
-package testutils
-
-import (
-	"net/http"
-	"net/http/httptest"
-
-	"github.com/mattermost/mattermost-server/services/httpservice"
-)
-
-type MockedHTTPService struct {
-	Server *httptest.Server
-}
-
-func MakeMockedHTTPService(handler http.Handler) *MockedHTTPService {
-	return &MockedHTTPService{
-		Server: httptest.NewServer(handler),
-	}
-}
-
-func (h *MockedHTTPService) MakeClient(trustURLs bool) *httpservice.Client {
-	return &httpservice.Client{Client: h.Server.Client()}
-}
-
-func (h *MockedHTTPService) Close() {
-	h.Server.CloseClientConnections()
-	h.Server.Close()
-}