Adding some return types

[gagoar]

per #38 I had to add some return types and be more specific about callbacks in mocks.

[codecov]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #47   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          304       305    +1     
  Branches        49        49           
=========================================
+ Hits           304       305    +1     

Flag	Coverage Δ
#unittests	100.00% <100.00%> (ø)

Impacted Files	Coverage Δ
src/__tests__/helpers.ts	100.00% <100.00%> (ø)
src/utils/setAWSCredentials.ts	100.00% <100.00%> (ø)
src/utils/tables.ts	100.00% <100.00%> (ø)

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