Adds client certificate authentication support

[Badgerati]

Description of the Change

Adds client certificate authentication support, as well as just generally allowing client certificates to be sent on requests and retrieved.

There's a new -AllowClientCertificate switch on Add-PodeEndpoint for HTTPS endpoints. When supplied, any requests to the server will accept a client certificate to be sent. This certificate is then available in the Request object under ClientCertificate, which is accessible from the current web event.

There's also a new -ClientCertificate switch on New-PodeAuthScheme; the appropriate scriptblock from Add-PodeAuth will be supplied with the client's certificate and any errors.

Related Issue

Resolves #472

Examples

Start-PodeServer {
    Add-PodeEndpoint -Address * -Port 8443 -Protocol Https -SelfSigned -AllowClientCertificate

    New-PodeAuthScheme -ClientCertificate | Add-PodeAuth -Name 'Login' -Sessionless -ScriptBlock {
        param($cert, $errors)

        # check if the client's cert is valid

        return @{ User = $user }
    }
}

By default, Pode will validate the certificate is present, and the Before/After dates - if not valid a 401 response is returned.