Expose JsonSerializerSettings in ApiClient

[davidyee]

In this PR, I expose the JsonSerializerSettings in the ApiClient so that serialization customizations can be done. For example, in my specific use-case, I wanted to add additional Converters to change the way JSON was being deserialized back into the csharp classes.

Fixes #4241

I reviewed the comments made on this related PR #4466 and incorporated the feedback into this PR.

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02)

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[wing328]

@davidyee thanks for the PR. I wonder if you can add a new ApiClientTests.cs file to test JsonSerializerSettings

A good reference is ConfigurationTests.cs

Ref: https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools.Test/ConfigurationTests.cs

[wing328]

FYI. I've filed #7583 to default OverrideSpecifiedNames to false. Let me know if you've any concerns.

[davidyee]

@wing328 Thanks for the feedback and suggestions. I've included revisions to my PR in my latest commits.

[wing328]

@davidyee thanks for the PR, which has been merged into master.