Add AddSystemd() and AddWindowsService() IServiceCollection extension methods (#68580)

Add AddSystemd() and AddWindowsService() IServiceCollection extension methods (#68580)

* Add AddSystemd() IServiceCollection extension method

* Add AddWindowsService() IServiceCollection extension method

* Don't default to CWD if in C:\Windows\system32
- instead, when CWD is C:\Windows\system32 Hosting will use AppContext.BaseDirectory. This way Windows apps and services that are launched will work by default. HostApplicationBuilder.ContentRootPath can't be changed after construction, so setting it to a workable default for Windows apps.

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Co-authored-by: Martin Costello <martin@martincostello.com>
* Use RemoteExecutor

* Update src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/HostTests.cs

* Skip test on Windows nano server

* Respond to PR feedback

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Co-authored-by: Martin Costello <martin@martincostello.com>