add "Get" method for es client

[mrparkers]

needed for rode's ListGenericResourceVersions rpc

[codecov-commenter]

Codecov Report

Merging #81 (cdb8dfd) into main (5c2d664) will decrease coverage by 0.15%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
- Coverage   85.90%   85.74%   -0.16%     
==========================================
  Files          11       11              
  Lines        1220     1235      +15     
==========================================
+ Hits         1048     1059      +11     
- Misses        108      110       +2     
- Partials       64       66       +2     

Impacted Files	Coverage Δ
go/v1beta1/storage/esutil/client.go	81.81% <73.33%> (-0.59%)	⬇️

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