Allow adjust start U number of numbered rack

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• [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe. When designing a cabinet, we want to split rack space into 3x15U or split it by function(power, compute, networking). The current numbered rack doesn't support adjust the start of the U number.

Describe the solution you'd like Add a property to numbered rack, to specify start rack start number.

Describe alternatives you've considered Don't split rack, but it mess large rack design.

Additional context