Redesign InMemoryExporter

1. Remove the idea of cloning the exporter as shown in https://github.com/open-telemetry/opentelemetry-rust/pull/2649 
2. Make it easier to construct InMemoryExporter easily and retrieve the items from it, instead of forcing users to hand create Mutex wrapped structs etc.
3. Need this for all signals, but would be easy to start with one and expand.

