Add missing methods in `Serializers` for Enum and TreeNode types (3.0)

[cowtowncoder]

Compared to Deserializers methods, Serializers is missing callbacks to 2 special types:

• Enums
• "tree node" subtypes (TreeNode)

and generic Serializers.findSerializer() (since no more specific callback exists) is called.

In addition, Deserializers uses too specific limit for "tree nodes" (JsonNode, should be TreeNode).

Since adding new callbacks is major backwards-compatibility change with some possibility of breakage, let's add callbacks only in 3.0 (ditto for signature change for Deserializers.findTreeNodeDeserializer()