feat: add ConfigSource field into ConsumerGroupPlugin

This commit is adding a new `ConfigSource` field into the
`ConsumerGroupPlugin` struct. This is not a field that should
be used for online operation, but it's needed by the
the `go-database-reconciler` to allow decK to work with
plugins config deduplication with Consumer Groups as well.

Ref: https://docs.konghq.com/deck/latest/guides/deduplicate-plugin-configuration/

kong/consumer_group.go

@@ -41,6 +41,7 @@ type ConsumerGroupPlugin struct {
 	CreatedAt     *int64         `json:"created_at,omitempty" yaml:"created_at,omitempty"`
 	Config        Configuration  `json:"config,omitempty" yaml:"config,omitempty"`
 	ConsumerGroup *ConsumerGroup `json:"consumer_group,omitempty" yaml:"consumer_group,omitempty"`
+	ConfigSource  *string        `json:"_config,omitempty" yaml:"_config,omitempty"`
 }
 
 // FriendlyName returns the endpoint key name or ID.