Blender Git Loki

Nodes: refactor socket declarations

This commits adds a few common flags to `SocketDeclaration`
so that they are available for all socket types (hide label, hide
value, is multi input). This allows porting over the remaining
geometry nodes to the new declaration system.

Furthermore, this commit separates the concepts of the socket
declaration and corresponding builders. The builders are used
by nodes to declare which sockets they have (e.g. `FloatBuilder`).
The ready build socket declarations can then be consumed by
other systems such as the versioning code. Both use cases
need different APIs and those will change for independent reasons,
so it makes sense to separate the classes.