[MM-52924] Implement ConfigurationWillBeSaved plugin hook (#23567)

* Implement ConfigurationWillBeSaved plugin hook

* Add comment

* Update comment

* Fix potential nil dereference if plugin environment is unset

* Address PR review

server/channels/app/plugin_api_tests/manual.test_configuration_will_be_saved_hook/main.tmpl

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+)
+
+type TestPlugin struct {
+	plugin.MattermostPlugin
+}
+
+func (p *TestPlugin) OnActivate() error {
+	fmt.Println("activated")
+	return nil
+}
+
+// This acts like a template as the content of this file gets passed to
+// fmt.Sprintf to inject additional logic based on the test case.
+func (p *TestPlugin) ConfigurationWillBeSaved(newCfg *model.Config) (*model.Config, error) {
+	%s
+}
+
+func main() {
+	plugin.ClientMain(&TestPlugin{})
+}