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[bot]]

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 :warning:
aws/dynamo/service.go	65.21%	4 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ 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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