Move plugins under Channels (#18938)

* Move plugins under Channels

We move all plugin related fields under *Channels.
This essentially migrates several methods from being under
*Server to under *Channels.

We also move the plugin startup and shutdown code
to be under Channels.Start and Channels.Shutdown.

While here, we remove the getPluginPublicKeyFiles
method which was a one-line method which uselessly
returned an error.

Lastly, we fix the product initialization order which
was incorrect previously. Products are dependent on
the main server.

So startup should be products -> server.
And shutdown should be server -> products.

```release-note
NONE
```

* Added app layer

```release-note
NONE
```

* Incorporate suggestions

```release-note
NONE
```

app/plugin_test.go

@@ -728,7 +728,7 @@ func TestPluginPanicLogs(t *testing.T) {
 		th.TestLogger.Flush()
 
 		// We shutdown plugins first so that the read on the log buffer is race-free.
-		th.App.Srv().ShutDownPlugins()
+		th.App.ch.ShutDownPlugins()
 		tearDown()
 
 		testlib.AssertLog(t, th.LogBuffer, mlog.LvlDebug.Name, "panic: some text from panic")
@@ -775,7 +775,7 @@ func TestProcessPrepackagedPlugins(t *testing.T) {
 			*cfg.PluginSettings.EnableRemoteMarketplace = false
 		})
 
-		plugins := th.App.Srv().processPrepackagedPlugins(prepackagedPluginsDir)
+		plugins := th.App.ch.processPrepackagedPlugins(prepackagedPluginsDir)
 		require.Len(t, plugins, 1)
 		require.Equal(t, plugins[0].Manifest.Id, "testplugin")
 		require.Empty(t, plugins[0].Signature, 0)
@@ -802,7 +802,7 @@ func TestProcessPrepackagedPlugins(t *testing.T) {
 
 		env := th.App.GetPluginsEnvironment()
 
-		plugins := th.App.Srv().processPrepackagedPlugins(prepackagedPluginsDir)
+		plugins := th.App.ch.processPrepackagedPlugins(prepackagedPluginsDir)
 		require.Len(t, plugins, 1)
 		require.Equal(t, plugins[0].Manifest.Id, "testplugin")
 		require.Empty(t, plugins[0].Signature, 0)
@@ -835,7 +835,7 @@ func TestProcessPrepackagedPlugins(t *testing.T) {
 		err = testlib.CopyFile(testPlugin2SignaturePath, filepath.Join(prepackagedPluginsDir, "testplugin2.tar.gz.sig"))
 		require.NoError(t, err)
 
-		plugins := th.App.Srv().processPrepackagedPlugins(prepackagedPluginsDir)
+		plugins := th.App.ch.processPrepackagedPlugins(prepackagedPluginsDir)
 		require.Len(t, plugins, 2)
 		require.Contains(t, []string{"testplugin", "testplugin2"}, plugins[0].Manifest.Id)
 		require.NotEmpty(t, plugins[0].Signature)
@@ -884,7 +884,7 @@ func TestProcessPrepackagedPlugins(t *testing.T) {
 		err = testlib.CopyFile(testPlugin2SignaturePath, filepath.Join(prepackagedPluginsDir, "testplugin2.tar.gz.sig"))
 		require.NoError(t, err)
 
-		plugins := th.App.Srv().processPrepackagedPlugins(prepackagedPluginsDir)
+		plugins := th.App.ch.processPrepackagedPlugins(prepackagedPluginsDir)
 		require.Len(t, plugins, 2)
 		require.Contains(t, []string{"testplugin", "testplugin2"}, plugins[0].Manifest.Id)
 		require.NotEmpty(t, plugins[0].Signature)
@@ -921,7 +921,7 @@ func TestProcessPrepackagedPlugins(t *testing.T) {
 		err = testlib.CopyFile(testPlugin2SignaturePath, filepath.Join(prepackagedPluginsDir, "testplugin2.tar.gz.sig"))
 		require.NoError(t, err)
 
-		plugins := th.App.Srv().processPrepackagedPlugins(prepackagedPluginsDir)
+		plugins := th.App.ch.processPrepackagedPlugins(prepackagedPluginsDir)
 		require.Len(t, plugins, 2)
 		require.Contains(t, []string{"testplugin", "testplugin2"}, plugins[0].Manifest.Id)
 		require.NotEmpty(t, plugins[0].Signature)