Add search indexing for dataset_description DatasetType field

[nellh]

Defaults to raw if unspecified by the dataset. Any string is allowed here for future BEPS extending this field if needed.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 42.50%. Comparing base (059e3b5) to head (589a8b1).

Files	Patch %	Lines
...ckages/openneuro-server/src/datalad/description.ts	50.00%	3 Missing ⚠️
packages/openneuro-server/src/graphql/schema.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3035   +/-   ##
=======================================
  Coverage   42.50%   42.50%           
=======================================
  Files         600      600           
  Lines       37906    37915    +9     
  Branches     1111     1113    +2     
=======================================
+ Hits        16112    16116    +4     
- Misses      21578    21583    +5     
  Partials      216      216           

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