SignedCMS.ComputeSignature change for .NET Core

[gewarren]

Fixes #3456

[gewarren]

Do we need to add in this info?

"It does look like it's only respected when the Windows PAL is used, so Windows OS without using options that force it into the managed PAL (like providing your own private key instance)."

[bartonjs]

Maybe the .NET Core value should be something like

true to request opening keys with PIN prompts disabled, where supported; otherwise, false. Then maybe a remark like

The silent parameter has no effect if the CmsSigner.PrivateKey value is not null; whether or not computing the signature will prompt is controlled by the provided value. Even when the PrivateKey property is null, some combinations of Operating System and signer options can result in a PIN prompt even when the silent parameter is set to true.

[gewarren]

Close/reopen

[tdykstra]

LGTM