fix(util-dynamodb): create BigInt from string value instead of number

[trivikr]

Issue #, if available:
Fixes: #1543

Description of changes:
creates BigInt from string value instead of number

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[codecov-commenter]

Codecov Report

Merging #1544 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1544      +/-   ##
==========================================
+ Coverage   79.84%   79.93%   +0.08%     
==========================================
  Files         304      306       +2     
  Lines       11753    11804      +51     
  Branches     2511     2527      +16     
==========================================
+ Hits         9384     9435      +51     
  Misses       2369     2369              

Impacted Files	Coverage Δ
packages/middleware-host-header/src/index.ts	80.00% <ø> (ø)
packages/util-dynamodb/src/convertToNative.ts	100.00% <100.00%> (ø)
packages/util-dynamodb/src/unmarshall.ts	100.00% <100.00%> (ø)

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[alexforsyth]

Looks good to me!

[github-actions]

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