schemeshard: forward declare events in a core header for operations (#11655)

Switch `schemeshard_operation_part.h` to forward declarations of component events.
Move component specific header includes from `schemeshard_operation_part.h` to relevant suboperation modules.

Event definitions often use component specific types and definitions which require dragging along component specific headers. As a result, any schemeshard suboperation module (which have to include base header `schemeshard_operation_part.h`) transitively include headers from all components for all scheme suboperations. Which is a bit excessive.

Made possible by #10685.

This is part of "improve schemeshard operation build-time" effort (#10633).

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