[BUG] HandlebarsEngineAdapter.java failed to use custom FieldValueResolver causing IllegalAccessException for maps

[WouterBaeyens]

The old MY_FIELD_VALUE_RESOLVER.INSTANCE is equivalent to FieldValueResolver.INSTANCE, which is assigned new FieldValueResolver(); so our custom class was never used.

This should improve #510 somewhat, and finally properly applies the suggestion from jknack/handlebars.java#940 (comment)

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[wing328]

@WouterBaeyens thanks for the PR

would it be possible to add a test or 2 in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/templating/HandlebarsEngineAdapterTest.java ?

[WouterBaeyens]

I added the test and did a little bit of clean-up as well.

• I have split off AccessAwareFieldValueResolver
  • It keeps the format aligned with the usage of the INSTANCE singleton
  • It enables us in test-methods like HandlebarsEngineAdapterTest to test using the resolver actually active in the code.
• I re-ordered the MethodValueResolver to take priority.
  • This is what was suggested by the maintainers of the handlebars repo
  • Map, JavaBean, Method, Value order makes more sense. i.e. getKey() > key() > key especially after the introduction of records is more logical over getKey() > key > key().

@wing328 let me know if you feel the tests are insufficient or further discussion is warranted. I realize I've slightly increased the scope of my change, but with increased understanding I've become pretty confident on the logical order of the resolvers.

[WouterBaeyens]

ping @wing328
Are the changes too complex to review with limited time-resources?
If you want I can revert the subsequent changes and create a follow-up mr for those.
I would like to help make handlebars a viable alternative and fixing partial templates is I think a requirement to do this.

Maybe there are people specific to handlebars I could tag instead?

[wing328]

thanks for the PR. let's give it a try