[MM-30436] Add support for configuration custom defaults (#16265)

* [MM-30436] Add support for configuration custom defaults

* Addressing review comments

* Addressing PR comments

* Soft fail if the configuration defaults cannot be read

* Directly print error in log message

* Fix linter

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

app/options.go

@@ -8,6 +8,7 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/mattermost/mattermost-server/v5/config"
+	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/store"
 )
 
@@ -38,10 +39,13 @@ func StoreOverride(override interface{}) Option {
 	}
 }
 
-// Config applies the given config dsn, whether a path to config.json or a database connection string.
-func Config(dsn string, watch bool) Option {
+// Config applies the given config dsn, whether a path to config.json
+// or a database connection string. It receives as well a set of
+// custom defaults that will be applied for any unset property of the
+// config loaded from the dsn on top of the normal defaults
+func Config(dsn string, watch bool, configDefaults *model.Config) Option {
 	return func(s *Server) error {
-		configStore, err := config.NewStore(dsn, watch)
+		configStore, err := config.NewStore(dsn, watch, configDefaults)
 		if err != nil {
 			return errors.Wrap(err, "failed to apply Config option")
 		}