Add hard delete function to multi_process_shared

[damccorm]

Adds an unsafe delete option to be used for garbage collection of multi_process_shared objects. This allows cleanup of multiprocess objects even if some references have been entirely lost (e.g. due to running in a subprocess mode) or if its unclear when they'll be invoked again (because Beam may not schedule work to that DoFn for a bit).

This is intentionally unsafe and should only be used when other synchronization mechanisms are in place to prevent usage of the already deleted object. For example, this will be used as part of our runinference timeout feature - for more details see https://docs.google.com/document/d/19ves6iv-m_6DFmePJZqYpLm-bCooPu6wQ-Ti6kAl2Jo/edit#heading=h.cfdh3ijyabi3

Part of #32137

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[jrmccluskey]

LGTM