Start populating/using *Asset.dataType ?

ATM we only have
```python
# this is from C2M2 level 1 - using EDAM vocabularies - in our case we would
# need to come up with things for neurophys
# TODO: waiting on input
dataType: Optional[HttpUrl] = Field(None, nskey="dandi")
```
more specifically this portion of that PR is a relevant discussion: https://github.com/dandi/dandi-cli/pull/226#issuecomment-683816377 and it relates to the spreadsheet @bendichter established recently (yet to undig url). Otherwise AFAIK that field is nowhere used/tested etc ATM.

With that in mind
- I guess it should also be RFed to not be just a pure HttpUrl but some "human accessible" vocabulary term and us to establish mapping, or how do you envision such cases @satra?
- should be a list and not a singular item since NWB can contain a number of dataType's

Since we are auto-extracting assets metadata for NWB, and gearing up to do that for BIDS datasets (Ref: https://github.com/dandi/dandi-cli/issues/432) we should be in good position to ensure reasonable coverage (of vocab terms) given the data we have at hands.