Allow customizing attribute types

[fehnomenal]

I needed to customize the type returned to graphql.

[codecov]

Codecov Report

Merging #282 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #282   +/-   ##
=========================================
  Coverage     74.08%   74.08%           
  Complexity     1072     1072           
=========================================
  Files            72       72           
  Lines          4765     4765           
  Branches        681      681           
=========================================
  Hits           3530     3530           
  Misses          929      929           
  Partials        306      306           

Impacted Files	Coverage Δ	Complexity Δ
...jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java	90.76% <ø> (ø)	141.00 <0.00> (ø)

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