[esutil.Client] support working with indices that use join fields

[mrparkers]

two significant changes here:

1) BulkCreate has been renamed to Bulk, so it can be used for indexing, creating, updating, and deleting. a new type was added for the BulkRequestItem so you can specify what each operation is. 2) Added a EsJoin type that can be used within Create and Bulk. this type has a custom JSON marshaller that allows you to specify join fields as a part of the document source without having to modify the original proto message.

more info on the join field here: https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html

no unit tests yet

[codecov-commenter]

Codecov Report

Merging #82 (e279941) into main (1df2c81) will decrease coverage by 0.30%. The diff coverage is 81.08%.

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   84.59%   84.28%   -0.31%     
==========================================
  Files           8        9       +1     
  Lines         954      999      +45     
==========================================
+ Hits          807      842      +35     
- Misses         88       93       +5     
- Partials       59       64       +5     

Impacted Files	Coverage Δ
go/v1beta1/storage/filtering/filtering.go	73.87% <ø> (ø)
go/v1beta1/storage/esutil/types.go	69.23% <69.23%> (ø)
go/v1beta1/storage/esutil/client.go	81.60% <76.74%> (-0.21%)	:arrow_down:
go/v1beta1/storage/elasticsearch.go	94.54% <100.00%> (+0.02%)	:arrow_up:

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