Use partitionKey from context in FixedWindowDynamoDB

[mennanov]

Add support for the partitionKey in the FixedWindowDynamoDB backend to be provided per-request:

window.Limit(l.NewFixedWindowDynamoDBContext(context.Background(), "anotherPartitionKey"))

Resolves #89

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.06%. Comparing base (d0f9793) to head (1a725ff).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   81.23%   81.06%   -0.18%     
==========================================
  Files          12       12              
  Lines        1711     1716       +5     
==========================================
+ Hits         1390     1391       +1     
- Misses        245      248       +3     
- Partials       76       77       +1     

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