MM-40818: Pass filestore to channels (#19677)

We move away from referencing server struct
but pass the filestore service to Channels
explicitly.

https://mattermost.atlassian.net/browse/MM-40818

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server.go

@@ -89,8 +89,9 @@ var SentryDSN = "placeholder_sentry_dsn"
 type ServiceKey string
 
 const (
-	ConfigKey  ServiceKey = "config"
-	LicenseKey ServiceKey = "license"
+	ConfigKey    ServiceKey = "config"
+	LicenseKey   ServiceKey = "license"
+	FilestoreKey ServiceKey = "filestore"
 )
 
 type Server struct {
@@ -362,8 +363,9 @@ func NewServer(options ...Option) (*Server, error) {
 	}
 
 	serviceMap := map[ServiceKey]interface{}{
-		ConfigKey:  s.configStore,
-		LicenseKey: s.licenseWrapper,
+		ConfigKey:    s.configStore,
+		LicenseKey:   s.licenseWrapper,
+		FilestoreKey: s.filestore,
 	}
 	// Step 8: Initialize products.
 	// Depends on s.httpService.