This patch adds support for _Menu Switch_ nodes and enum definitions in
node trees more generally. The design is based on the outcome of the
[2022 Nodes Workshop](https://code.blender.org/2022/11/geometry-nodes-workshop-2022/#menu-switch).
The _Menu Switch_ node is an advanced version of the _Switch_ node which
has a customizable **menu input socket** instead of a simple boolean.
The _items_ of this menu are owned by the node itself. Each item has a
name and description and unique identifier that is used internally. A
menu _socket_ represents a concrete value out of the list of items.
To enable selection of an enum value for unconnected sockets the menu is
presented as a dropdown list like built-in enums. When the socket is
connected a shared pointer to the enum definition is propagated along
links and stored in socket default values. This allows node groups to
expose a menu from an internal menu switch as a parameter. The enum
definition is a runtime copy of the enum items in DNA that allows
sharing.
A menu socket can have multiple connections, which can lead to
ambiguity. If two or more different menu source nodes are connected to a
socket it gets marked as _undefined_. Any connection to an undefined
menu socket is invalid as a hint to users that there is a problem. A
warning/error is also shown on nodes with undefined menu sockets.
At runtime the value of a menu socket is the simple integer identifier.
This can also be a field in geometry nodes. The identifier is unique
within each enum definition, and it is persistent even when items are
added, removed, or changed. Changing the name of an item does not affect
the internal identifier, so users can rename enum items without breaking
existing input values. This also persists if, for example, a linked node
group is temporarily unavailable.
Pull Request: https://projects.blender.org/blender/blender/pulls/113445
5ad49f4142