docstore/awsdynamodb: support the AWS v2 SDK

[vangent]

Cloned from #3519.

[codecov]

Codecov Report

Attention: Patch coverage is 74.87923% with 260 lines in your changes missing coverage. Please review.

Project coverage is 70.21%. Comparing base (ad1e769) to head (da27b7d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
docstore/awsdynamodb/v2/dynamo.go	73.17%	82 Missing and 39 partials ⚠️
docstore/awsdynamodb/v2/codec.go	63.84%	53 Missing and 24 partials ⚠️
docstore/awsdynamodb/v2/query.go	83.28%	45 Missing and 17 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3602      +/-   ##
==========================================
+ Coverage   69.90%   70.21%   +0.31%     
==========================================
  Files         114      118       +4     
  Lines       14699    15779    +1080     
==========================================
+ Hits        10276    11080     +804     
- Misses       3694     3888     +194     
- Partials      729      811      +82     

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