refactor: adopt same code conventions as kafkaflow

[gsferreira]

Description

As discussed with @luispfgarces, this PR applies the same code conventions adopted by KafkaFlow v3 (https://github.com/Farfetch/kafkaflow/pull/475 and https://github.com/Farfetch/kafkaflow/pull/485).

• move .sln file to root
• move test projects to tests folder
• use _ as private field prefix
• global usings
• file-scoped namespaces

How Has This Been Tested?

Run automated tests.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have added tests to cover my changes
• [ ] I have made corresponding changes to the documentation

Disclaimer

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[gsferreira]

@luispfgarces rebased.