Polarized SANS metadata: add user file definition and ability to parse inputs

[rbauststfc]

Part of #36147. See the design and requirements documents, and the NXcanSAS specification proposal, that are linked on that issue for more details of what needs to be implemented.

To allow polarized SANS metadata to be saved to the NXcanSAS file via the ISIS SANS reduction workflow, we need to support polarized SANS settings in the TOML user file (also referred to as a mask file). Refer to the IDF components and User File Definition sections of the design document for the specific details of the settings we have agreed need to be implemented.

This issue will be limited to extending the user file parser and state object so that we can read in user files with these settings, apply appropriate validation and store the values in the state object for later use. Actually using the values as part of the reduction workflow save step will be implemented under a separate issue.

A couple of considerations for implementation:

• If documentation is added under this issue (rather than at a later stage) then we should make it clear that the new settings won't actually have any effect yet.
• The user file definition allows efficiency, transmission or empty cell data to be provided by workspace name or by file name. If a file name/path is provided then we will attempt to load the file into a workspace to pass to the save algorithm. We could potentially do this later in the reduction workflow, but it's likely to be more user friendly to load the workspaces as part of parsing the file so that if this fails for any reason then the user can be made aware early on (the full reduction workflow will take some time, so finding out about this right at the end would not be ideal). If we did this then we would obviously need to check the impact on performance of parsing of the user file, though.