Azure Key Vault: fix regression in Azure Key Vault signing

[dtivel]

Resolve #827

#808 and #822 introduced a regression.

In Azure Key Vault:

• a Key Vault has a unique URI of the form: <scheme>://<authority>
• a certificate has a unique URI of the form: <scheme>://<authority>/certificates/<certificateId>
• a private key has a unique URI of the form: <scheme>://<authority>/keys/<certificateId>

#808 and #822 added calls to CertificateClientBuilderExtensions.AddCertificateClient(...) and KeyClientBuilderExtensions.AddCryptographyClient(...). AddCertificateClient(...) has a vaultUri parameter which must be the Key Vault URI, while AddCryptographyClient(...) has a vaultUri parameter which must be the private key's URI. Documentation for the latter says, vaultUri should be "[t]he URI to a specific key in an Azure Key Vault, for example: https://my-vault.vault.azure.net/keys/my-key."

However, KeyVaultKeyIdentifier.VaultUri was passed to AddCryptographyClient(...), and that returns the Key Vault URI. The fix is to use the private key URI we already constructed ourselves.