Make execution strategies configurable

[fehnomenal]

I wanted to specify my own DataFetcherExceptionHandler so here we are to allow define custom ExecutionStrategies.

[codecov]

Codecov Report

Merging #281 into master will increase coverage by 0.02%.
The diff coverage is 78.57%.

@@             Coverage Diff              @@
##             master     #281      +/-   ##
============================================
+ Coverage     74.08%   74.10%   +0.02%     
  Complexity     1072     1072              
============================================
  Files            72       72              
  Lines          4765     4792      +27     
  Branches        681      681              
============================================
+ Hits           3530     3551      +21     
- Misses          929      935       +6     
  Partials        306      306              

Impacted Files	Coverage Δ	Complexity Δ
.../schema/impl/GraphQLJpaExecutorContextFactory.java	67.30% <50.00%> (-5.20%)	12.00 <0.00> (ø)
...utoconfigure/GraphQLJpaQueryAutoConfiguration.java	100.00% <100.00%> (ø)	1.00 <0.00> (ø)
...a/query/schema/impl/GraphQLJpaExecutorContext.java	100.00% <100.00%> (ø)	11.00 <0.00> (ø)

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