MM-14575 - Automatically serve static files for plugins (#10476)

* MM-14575 - Automatically serve static files for plugins
* Added static handler for plugin public files
* Added StaticFilesPath method to Environment for use by MainRouter
* Added "static_files" property to Manifest Server
* Added unit tests for these changes

* MM-14575: Adding comment for cache control value

* MM-14575: Moved Static Plugin Request handler to plugin_requests
* Updated testing

* MM-14575: Removing the StaticFiles from Manifest Server

* MM-14575: Removing static files from test

* MM-14575: Updating static files test

* MM14575: Removing cache directive from plugin static files

* MM14575: Moving plugin public directory to root

* MM-14575: Updating tests for changed public directory

* MM-14575: Moved compileGo to a common utils package for tests

* MM-14575: Moving plugins initialization to InitPlugins find in tests

* Update utils/test_files_compiler.go

Adding Copyright header

Co-Authored-By: happygaijin <happygaijin@users.noreply.github.com>

* MM-14575: Consistent usage of static vs public name

* Removing spurious newline

* Comment typo

Co-Authored-By: happygaijin <happygaijin@users.noreply.github.com>

* Removing spurious new line

Co-Authored-By: happygaijin <happygaijin@users.noreply.github.com>

* MM14575: Adding a test to make sure only public files can be requested

* MM-14575 Adding a test for redirects on public files

plugin/supervisor_test.go

@@ -6,12 +6,12 @@ package plugin
 import (
 	"io/ioutil"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"testing"
 
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/utils"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -26,18 +26,6 @@ func TestSupervisor(t *testing.T) {
 	}
 }
 
-func compileGo(t *testing.T, sourceCode, outputPath string) {
-	dir, err := ioutil.TempDir(".", "")
-	require.NoError(t, err)
-	defer os.RemoveAll(dir)
-	require.NoError(t, ioutil.WriteFile(filepath.Join(dir, "main.go"), []byte(sourceCode), 0600))
-	cmd := exec.Command("go", "build", "-o", outputPath, "main.go")
-	cmd.Dir = dir
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	require.NoError(t, cmd.Run())
-}
-
 func testSupervisor_InvalidExecutablePath(t *testing.T) {
 	dir, err := ioutil.TempDir("", "")
 	require.NoError(t, err)
@@ -83,7 +71,7 @@ func testSupervisor_StartTimeout(t *testing.T) {
 	defer os.RemoveAll(dir)
 
 	backend := filepath.Join(dir, "backend.exe")
-	compileGo(t, `
+	utils.CompileGo(t, `
 		package main
 
 		func main() {