Runtime side of https://github.com/paritytech/polkadot-sdk/issues/5048

Send the core selector ump signal in cumulus. Guarded by a compile time
feature until nodes are upgraded to a version that includes
https://github.com/paritytech/polkadot-sdk/pull/5423 for gracefully
handling ump signals.

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Co-authored-by: GitHub Action <action@github.com>