MM-12193: remove auto unmarshalling (#9519)

* MM-12193: remove auto configuration unmarshalling

Since plugin hook events are called concurrently, there's no way for the plugin framework to coordinate safe access to the automatically unmarshalled configuration fields. Remove this functionality, and update documentation to illustrate a safe way to do this.

* better Fprint example

* fix unit tests

* log when OnConfigurationChange fails through OnActivate

* clarify lifecycle when OnConfigurationChange returns an error

* call SetAPI even if OnConfigurationChange not implemented

app/plugin_api_test.go

@@ -76,16 +76,28 @@ func TestPluginAPILoadPluginConfiguration(t *testing.T) {
 			"fmt"
 		)
 
-		type MyPlugin struct {
-			plugin.MattermostPlugin
-			
+		type configuration struct {
 			MyStringSetting string
 			MyIntSetting int
 			MyBoolSetting bool
 		}
 
+		type MyPlugin struct {
+			plugin.MattermostPlugin	
+
+			configuration configuration
+		}
+
+		func (p *MyPlugin) OnConfigurationChange() error {
+			if err := p.API.LoadPluginConfiguration(&p.configuration); err != nil {
+				return err
+			}
+
+			return nil
+		}
+
 		func (p *MyPlugin) MessageWillBePosted(c *plugin.Context, post *model.Post) (*model.Post, string) {
-			return nil, fmt.Sprintf("%v%v%v", p.MyStringSetting, p.MyIntSetting, p.MyBoolSetting)
+			return nil, fmt.Sprintf("%v%v%v", p.configuration.MyStringSetting, p.configuration.MyIntSetting, p.configuration.MyBoolSetting)
 		}
 
 		func main() {
@@ -135,16 +147,28 @@ func TestPluginAPILoadPluginConfigurationDefaults(t *testing.T) {
 			"fmt"
 		)
 
-		type MyPlugin struct {
-			plugin.MattermostPlugin
-			
+		type configuration struct {
 			MyStringSetting string
 			MyIntSetting int
 			MyBoolSetting bool
 		}
 
+		type MyPlugin struct {
+			plugin.MattermostPlugin
+
+			configuration configuration
+		}
+
+		func (p *MyPlugin) OnConfigurationChange() error {
+			if err := p.API.LoadPluginConfiguration(&p.configuration); err != nil {
+				return err
+			}
+
+			return nil
+		}
+
 		func (p *MyPlugin) MessageWillBePosted(c *plugin.Context, post *model.Post) (*model.Post, string) {
-			return nil, fmt.Sprintf("%v%v%v", p.MyStringSetting, p.MyIntSetting, p.MyBoolSetting)
+			return nil, fmt.Sprintf("%v%v%v", p.configuration.MyStringSetting, p.configuration.MyIntSetting, p.configuration.MyBoolSetting)
 		}
 
 		func main() {