Add support to pull secrets from KeyVault using managed identities easier

[kalyankrishna1]

similar to support available for certificates as detailed here , Using certificates with Microsoft.Identity.Web, it'd be good if the same support was available for secrets as well.

An example follows:

"ClientSecrets": [
{
"SourceType": "KeyVault",
"KeyVaultUrl": "https://msidentitywebsamples.vault.azure.net",
"KeyVaultSecretName": "MicrosoftIdentitySamplesSecret"
}
]

Secrets , along with certificates are the two available forms of credentials for an app registered in azure AD today. Supporting certificated will help complete the all credentials out of code story better.

[jmprieur]

I propose to take this one with AzureAD/microsoft-authentication-library-for-dotnet#2596.

[jmprieur]

@jennyf19
This would actually simplify the tests of the devapps, samples, etc ...