Add sample for getting private key if exportable

[heaths]

Add a sample to get the private key in a secret, based on https://github.com/heaths/azsdk-sample-getcert.

• Link to it from the readme, e.g. https://docs.microsoft.com/dotnet/api/overview/azure/security.keyvault.certificates-readme?view=azure-dotnet#retrieve-a-certificate
• A link to it from the KeyVaultCertificate.Cer property