feat(document content): support external id and instance id

[andersfylling]

We recently deprecated the GET variant in favor of a POST variant as we plan to extend the request options in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.46%. Comparing base (297b56e) to head (bb38f03).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2046   +/-   ##
=======================================
  Coverage   90.46%   90.46%           
=======================================
  Files         139      139           
  Lines       22393    22399    +6     
=======================================
+ Hits        20257    20263    +6     
  Misses       2136     2136           

Files with missing lines	Coverage Δ
cognite/client/_api/documents.py	78.22% <100.00%> (+1.10%)	⬆️
cognite/client/_version.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[haakonvt]

LGTM