[PowerShell] Fix proxy type (@wing328)

The file configuration.mustache in modules/openapi-generator/src/main/resources/powershell/configuration.mustache contains the following check in the function Set-{{{apiNamePrefix}}}Configuration

If ($Proxy.GetType().FullName -ne "System.Net.SystemWebProxy" -and $Proxy.GetType().FullName -ne "System.Net.WebRequest+WebProxyWrapperOpaque") {
    throw "Incorrect Proxy type '$($Proxy.GetType().FullName)'. Must be System.Net.SystemWebProxy or System.Net.WebRequest+WebProxyWrapperOpaque."
}

The check fails in multiple environments, either because the type checked is unknown, or because the class returned by GetSystemWebProxy() is different from what is expected.

Since all returned types implement (and by the available doc, shall implement) the interface IWebProxy, I propose to replace the above test with

If ('System.Net.IWebProxy' -notin $Proxy.GetType().ImplementedInterfaces.FullName) {
    throw "Incorrect Proxy type '$($Proxy.GetType().FullName)'. Must implement System.Net.IWebProxy interface."
}

The change does not guarantee that the configured proxy will work on all systems (e.g. on Linux the default class is HttpNoProxy, and on MacOS proxy credentials are likely not yet implemented). Yet, when using the command

Set-PSConfiguration -Proxy ([system.net.webproxy]::new('http://127.0.0.1:8080')) -SkipCertificateCheck

the request and response are intercepted as requested in the proxy, and can be used for debugging.

The test can be made by using e.g. Burp Suite Community Edition, enabling the proxy on 127.0.0.1:8080 and using the line above to configure the PowerShell PetStore client API.

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