[🚀 Feature]: [dotnet] Revisit unit tests infrastructure

### Feature and motivation

Current situation is:


### Usage example

Concerns:
- It could be one test project because Chrome/Edge/Firefox drivers live in one project supported by Selenium (like built-in classes).
- Default namespace in test project is `OpenQA.Selenium` when usually it should be `OpenQA.Selenium.Tests`. I don't understand why it is unusual.
- Avoid any `static` context (`EnvironmentManager` class) - it will allow to look at parallelization. My computer has 6/12 cores, I want to utilize all of them when running Selenium unit tests.
- Provide a possibility to override driver options, like required in https://github.com/SeleniumHQ/selenium/pull/15850