Add Azure KeyVault cryptographic keys check

[Riff451]

Hi, this a proposed PR to slightly extend the already existing HealthChecks.AzureKeyVault created with PR #19 .
The idea is to support the check also for the cryptographic keys that can be stored in KeyVault. I noticed that another user suggested the change in the above PR, but I couldn't find any ongoing work.

Usage:

Default usage with Managed Service Identity:

services.AddHealthChecks()
    .AddAzureKeyVault(setup =>
    {
        setup
            .UseKeyVaultUrl("https://[vaultname].vault.azure.net/")                  
            .AddKey("keyname");
    });

Usage usage client id and client secret:

services.AddHealthChecks()
    .AddAzureKeyVault(setup =>
    {
        setup
            .UseKeyVaultUrl("https://[vaultname].vault.azure.net/")    
            .UseClientSecrets("clientid", "clientsecret")
            .AddKey("keyname");
    });

Thanks.

[CarlosLanderas]

Thanks @Riff451, let me test it and I'll merge it

[CarlosLanderas]

Tested and working. Thanks for the PR @Riff451

[CarlosLanderas]

Adding certificates feature as well:

#369

[Riff451]

Thanks @CarlosLanderas for the quick review and for the new certificates feature as well :)