fix: Memory leak on global IOC container and classValidatorMetadataStorage

[lmeysel]

Description

When using class-validator it is considered to act in a kind of "singleton" environment, sharing all the metadata once loaded. However e.g. when running integration tests (in my case for nest.js), this leads to memory leaks, as everything is loaded over and over again for each test suite. In #1236 it is declared as by design. However it would be nice to have some kind of cleanup method which resets all the global storages, and cleaning the instance array of the defaultContainer, as well as unsetting the userContainer