Add DynamoDB limiter backend

[lamebear]

This adds support to use DynamoDB as a Limiter backend for:

• FixedWindow
• SlidingWindow
• LeakyBucket
• TokenBucket

[codecov[bot]]

Codecov Report

Merging #13 (03b164e) into master (88e8f8b) will decrease coverage by 0.07%. The diff coverage is 82.88%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   83.08%   83.01%   -0.07%     
==========================================
  Files           9       10       +1     
  Lines         798     1166     +368     
==========================================
+ Hits          663      968     +305     
- Misses         92      135      +43     
- Partials       43       63      +20     

Impacted Files	Coverage Δ
dynamodb.go	67.90% <67.90%> (ø)
slidingwindow.go	86.70% <86.04%> (-0.80%)	:arrow_down:
tokenbucket.go	80.35% <86.41%> (+1.89%)	:arrow_up:
fixedwindow.go	91.26% <86.66%> (-3.57%)	:arrow_down:
leakybucket.go	80.26% <89.33%> (+2.97%)	:arrow_up:

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[mennanov]

Is it possible to improve test coverage for dynamodb.go?

[lamebear]

The dynamodb.go file is showing 81.1% coverage locally, 75% prior to this latest commit. Don't know why codecov is only showing 61% prior to this last commit. Any more tests on that file will be just for the sake of coverage percentage and not gaining value.

[mennanov]

Awesome! Thanks for your work!