Fix bug where the dynamoDBMapperRef config is not being read

[felixwimpyw]

The class dynamoDBTemplateBuilder is still getting reference from field dynamoDBMapperName, while when we set attribute dynamoDBMapperRef in EnableDynamoDBRepositories, field of dynamoDBMapperName will always null. It makes the application failed to startup with Error java.lang.IllegalArgumentException: 'beanName' must not be empty

Fixing by registering using the correct attribute.

[codecov-commenter]

Codecov Report

Merging #26 into develop will decrease coverage by 0.05%.
The diff coverage is 60.00%.

@@              Coverage Diff              @@
##             develop      #26      +/-   ##
=============================================
- Coverage      67.07%   67.01%   -0.06%     
  Complexity       664      664              
=============================================
  Files             84       84              
  Lines           2293     2295       +2     
  Branches         403      404       +1     
=============================================
  Hits            1538     1538              
- Misses           571      572       +1     
- Partials         184      185       +1     

Impacted Files	Coverage Δ	Complexity Δ
...tory/config/DynamoDBRepositoryConfigExtension.java	66.66% <60.00%> (-1.30%)	14.00 <0.00> (ø)

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