[mlir-opt] Enable defining which operations are defined at link time.

Previously mlir-opt had initializeMLIRContext function that added certain ops to the OperationSet of the context. But for different tests we'd want to register different ops. Make initializeMLIRContext an extern function so that the context initialization/set of ops to register can be determined at link time. This allows out-of-tree operations to easily expand the custom parsing/printing while still using mlir-opt.

PiperOrigin-RevId: 209078315