Add metadata on NWB extensions

[yarikoptic]

@satra mentioned Instruments . We should also add some UUID to identify that we are talking about "nwb extension".

We had #142 which was more overarching and also mentions layout (#157)

Based on that we can establish dashboard listing dandisests/assets and extensions, and probably point to example dandisets with those extensions from NWB extensions registry (an idea was welcomed by @bendichter )

• @satra to provide the "placement" metadata within dandi-schema
• @yarikoptic extraction within dandi-cli

[yarikoptic]

in #160 @bendichter already kindly provided the code to do such extraction from nwb files.