allow injection of custom bean introspection

[driabtchik]

It is rather common to implement POJOs that allow for chaining of setters (i.e. setters are returning "this").
Since BeanMapper relies on Introspector for extraction of properties, the property descriptors for such setters appear as "read-only", without populated setter method.
The change allows injection of customized method for bean introspection, while retaining compatibility with the original approach

[codecov]

Codecov Report

Merging #120 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
+ Coverage     93.76%   93.85%   +0.08%     
- Complexity      847      851       +4     
============================================
  Files            87       87              
  Lines          1781     1789       +8     
  Branches        173      173              
============================================
+ Hits           1670     1679       +9     
+ Misses           67       66       -1     
  Partials         44       44

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/io/beanmapper/dynclass/ClassStore.java	87.5% <100%> (ø)	5 <1> (ø)	⬇️
...n/java/io/beanmapper/config/BeanMapperBuilder.java	100% <100%> (ø)	42 <1> (+1)	⬆️
...in/java/io/beanmapper/dynclass/ClassGenerator.java	96.55% <100%> (ø)	19 <1> (ø)	⬇️
...o/beanmapper/core/inspector/PropertyAccessors.java	91.48% <100%> (+2.6%)	17 <1> (+2)	⬆️
...n/java/io/beanmapper/core/BeanPropertyCreator.java	100% <100%> (ø)	12 <1> (ø)	⬇️
...c/main/java/io/beanmapper/core/BeanMatchStore.java	96.82% <100%> (+0.05%)	37 <1> (+1)	⬆️
...n/java/io/beanmapper/config/CoreConfiguration.java	98.01% <100%> (+0.01%)	64 <0> (ø)	⬇️

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