Add basic get/put operations to dynamo service

[rowanseymour]

Also tweak .Test() to be a list tables operation instead of a describe table, because it gets used in tests where the table might not exist yet.

[codecov]

Codecov Report

Attention: Patch coverage is 65.95745% with 16 lines in your changes missing coverage. Please review.

Project coverage is 90.55%. Comparing base (2d72c4b) to head (955c31f).

Files with missing lines	Patch %	Lines
aws/dynamo/marshal.go	66.66%	5 Missing and 3 partials ⚠️
aws/dynamo/service.go	65.21%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
- Coverage   91.34%   90.55%   -0.80%     
==========================================
  Files          47       48       +1     
  Lines        1734     1779      +45     
==========================================
+ Hits         1584     1611      +27     
- Misses        105      115      +10     
- Partials       45       53       +8     

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