Enable "Set members..." support for vectors and similar structs

Currently, structs that have a custom make function aren't able to use the "Set members..." node in Blueprints. For vectors and similar structs, this is slightly inconvenient since it requires using a cumbersome workaround when setting individual fields. The "Set members..." node allows for a slightly more succinct solution.

Note that we need to be careful about which structs are permitted. By default, structs that have the HasNativeMake metadata won't use this node. There are some structs that either don't have a exposed properties (eg: FSoftObjectPath), or structs that have strange creation semantics (eg: FBox2D's use of bIsValid).

The preferred solution would create a new metadata tag (eg: DirectPropertyAccess or similar) that can be added to UK2Node_SetFieldsInStruct::CanBeMade.

Current:

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Proposed:
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• Create a new vector variable in a Blueprint.
• Drag the variable to the editor window and create a getter node.
• Drag a link from the vector variable pin.
• Search for "Set members..." in the Action menu.
• Observe that no option exists.