store probe set for image-based transcriptomics in spatialdata

[giovp]

after discussion with @LouisK92 and @timtreis we realized it would be useful for spatialdata to also store the probe set used to target mRNA transcripts. Where should that be stored, unclear, maybe uns?

[giovp]

additionaly, save also additional info such as add-on panel Supplemental: Add-on Panel (CSV)

[giovp]

• save json file in either dataframe or just json in table.uns
• store additional probe information in table.var

[LucaMarconato]

As discussed during the community call, we can start with var and see how the format from 10x evolves before adding the json dataframe in uns, as it would be a technology-specific way to add metadata and in general we should keep this to a minimum until we have a system taylored for metadata (@melonora).