Update API to allow QubitMapper where QubitConverter is used

[mrossinek]

What should we add?

This will be the first step of #967 because it can be largely parallelized with Qiskit/qiskit#9140.

The idea here is to update all occurrences of QubitConverter in the public API to also allow direct usage of QubitMappers. This will require potential isinstance checks to differentiate the two cases from another, but for the transition period while QubitConverter is being deprecated there is no way around this.

Until later parts of #967 are completed, some mappers will not provide the full functionality that is available through the QubitConverter but for the sake of not having to do all the changes at once in one massive PR this is okay.

Unittests should be added to assert the working cases.

[Anthony-Gandon]

I can contribute here