feat: add RetryStrategy class and retryMiddleware implementation

[AllanZhengYP]

Put retry logic into RetryStrategy itself. Keep retryMiddleware minimal.

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Codecov Report

Merging #389 into smithy-codegen will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           smithy-codegen     #389      +/-   ##
==================================================
+ Coverage           96.97%   96.97%   +<.01%     
==================================================
  Files                  69       69              
  Lines                1156     1158       +2     
  Branches              211      213       +2     
==================================================
+ Hits                 1121     1123       +2     
  Misses                 35       35

Impacted Files	Coverage Δ
packages/retry-middleware/src/defaultStrategy.ts	100% <100%> (ø)	⬆️
packages/retry-middleware/src/retryMiddleware.ts	77.77% <100%> (-14.23%)	⬇️

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