C#: Expose rid_allocate_id and rid_from_int64

[beicause]

Adds GD.RidAllocateId and GD.RidFromInt64 in C#.

rid_allocate_id and rid_from_int64 provide a way to create rid, but they are missing in C#.(https://docs.godotengine.org/en/latest/tutorials/scripting/c_sharp/c_sharp_differences.html)

[raulsntos]

The documentation of these methods say they are mainly used from native extensions to implement servers, which is not an use case supported with C# as a scripting language. So I'm wondering about the specific need for these methods from C#, could you share your specific use case?

[beicause]

They are indeed rarely used in scripts. One use case I think is deralization of Rid, because the Id field of Rid is meaningless without rid_from_int64.

[raulsntos]

I don't think it makes sense to serialize RIDs, Godot always serializes them as RID(). The RIDs assigned to your resources are not guaranteed to stay the same on every execution, see this note from the RID documentation:

Note: RIDs are only useful during the current session. It won't correspond to a similar resource if sent over a network, or loaded from a file at a later time.

[beicause]

As long as the rid is not released, it is still accessible, and I think this should be allowed at runtime, for example serializing a custom struct that contains an rid (although it is often unnecessary to do so). I also found although the documentation states that rid is not supported serialization, var_to_bytes and bytes_to_var can correctly handle rid (godotengine/godot-docs#10662).