[feat] Use `fields` to directly filter response data

[hongyuanjia]

Pull request overview

• Fixes #66

[codecov-commenter]

Codecov Report

Merging #72 (d54da66) into master (04ce88c) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   99.81%   99.82%           
=======================================
  Files           7        7           
  Lines        1654     1678   +24     
=======================================
+ Hits         1651     1675   +24     
  Misses          3        3           

Impacted Files	Coverage Δ
R/esgf.R	100.00% <100.00%> (ø)

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