Support metalbond client creation with proper Context handling

[afritzler]

Summary

Currently we instantiate the metalbond instance using

mb := metalbond.NewMetalBond(config, metalbondClient)

and initiate a shutdown of the instance via

mb.Shutdown()

It would be better if we would have a contractor method with a proper context. E.g.

mb := metalbond.NewMetalBondWithContext(ctx, config, metalbondClient)

Alternatively we can keep constructor method as is and have a Start(ctx context.Context) method to start the metalbond instance.