Chore: Remove public vars in setting package (#81018)

Removes the public variable setting.SecretKey plus some other ones. 
Introduces some new functions for creating setting.Cfg.

pkg/api/pluginproxy/pluginproxy_test.go

@@ -28,7 +28,6 @@ import (
 )
 
 func TestPluginProxy(t *testing.T) {
-	setting.SecretKey = "password"
 	secretsService := secretsManager.SetupTestService(t, fakes.NewFakeSecretsStore())
 
 	t.Run("When getting proxy headers", func(t *testing.T) {