WSIReader support for reading by existing power and mpp #6289

In MONAI Pathology WG, we have discussed about the need for being able to read whole slide images at specific resolution (mpp or magnification) in addition to reading at a specific level.
As discussed in pathology working group, this feature request is part of #4980 that defines the need to read the existing objective power and magnification without interpolating if the requested power or mpp does not exist. We decided to address this part first as the interpolation part requires some discussion on the details.