MM-40818: Refactor product initialization to happen late (#19658)

This is more of a general refactor of the initialization
process which should allow us to pass services more
easily.

The changes are minimal to keep the scope limited.
For now, the objective is to pass the file service
to the Channels product. For that, it was required
to move some of the enterprise interfaces under Channels
from Server.

We also create a filestore field in the server to
avoid creating filestore reference every time
we make a filestore operation. This will be later
passed on to the Channels product.

Also removed an unnecessary test. 
The test was working so far because we were creating
the filebackend every time for every request. But
we should go via UpdateConfig call which would fail,
were we to assign an invalid filestore name.

So we were actually testing for a different thing.
Therefore, removed the test.

```release-note
NONE
```

app/channels.go

@@ -75,6 +75,9 @@ type Channels struct {
 	Compliance       einterfaces.ComplianceInterface
 	DataRetention    einterfaces.DataRetentionInterface
 	MessageExport    einterfaces.MessageExportInterface
+	Saml             einterfaces.SamlInterface
+	Notification     einterfaces.NotificationInterface
+	Ldap             einterfaces.LdapInterface
 
 	// These are used to prevent concurrent upload requests
 	// for a given upload session which could cause inconsistencies
@@ -145,6 +148,24 @@ func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, err
 	if accountMigrationInterface != nil {
 		ch.AccountMigration = accountMigrationInterface(New(ServerConnector(ch)))
 	}
+	if ldapInterface != nil {
+		ch.Ldap = ldapInterface(New(ServerConnector(ch)))
+	}
+	if notificationInterface != nil {
+		ch.Notification = notificationInterface(New(ServerConnector(ch)))
+	}
+	if samlInterfaceNew != nil {
+		ch.Saml = samlInterfaceNew(New(ServerConnector(ch)))
+		if err := ch.Saml.ConfigureSP(); err != nil {
+			mlog.Error("An error occurred while configuring SAML Service Provider", mlog.Err(err))
+		}
+
+		ch.AddConfigListener(func(_, _ *model.Config) {
+			if err := ch.Saml.ConfigureSP(); err != nil {
+				mlog.Error("An error occurred while configuring SAML Service Provider", mlog.Err(err))
+			}
+		})
+	}
 
 	var imgErr error
 	ch.imgDecoder, imgErr = imaging.NewDecoder(imaging.DecoderOptions{