Polarized SANS metadata: extend LoadNXcanSAS to load polarized NXcanSAS data

[rbauststfc]

Part of #36147. See that issue for the context of what is required.

Once we have added a SavePolarizedNXcanSAS algorithm for saving out polarized SANS data, we will also need to provide the functionality for the data to be loaded back into Mantid. This will mean extending/updating the LoadNXcanSAS algorithm.

I've only tested one file, but the current reloading of NXcanSAS files looks like it loads the data but not much metadata from the file. The instrument is not loaded onto the workspace, for example, and the sample logs are minimal. So a sensible first step may be to make sure we can load multiple data blocks and not look at loading any additional metadata into the workspace.

We have not yet discussed the loading of reduced polarized SANS data with our scientists. I would recommend that if they would like any of the polarized SANS metadata to be loaded back into the workspace from the NXcanSAS file then we handle this under separate issues and discuss whether it forms part of the epic MVP or not.