Add KeyVault Certificate Functions (#334)

* added certificate.rs

* rename exp to expires on

* addressed heaths comments in PR

* added in certificate client

* Revert "added in certificate client"

This reverts commit dc4c8bc.

* fixing mismatched rebase

* fixed bugs in cargo tests

* fixed cargo test errors

* impled CertificateClient

* fixed integration tests (again)

* addressed heaths reqeusted changes

* removed errant std fmt use

Author: Billy-Sheppard

sdk/security_keyvault/examples/update_secret.rs

@@ -1,5 +1,5 @@
 use azure_identity::token_credentials::{ClientSecretCredential, TokenCredentialOptions};
-use azure_security_keyvault::{KeyClient, RecoveryLevel};
+use azure_security_keyvault::KeyClient;
 use chrono::prelude::*;
 use chrono::Duration;
 use std::env;
@@ -31,7 +31,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     // Update secret recovery level to `Purgeable`.
     client
-        .update_secret_recovery_level(&secret_name, &secret_version, RecoveryLevel::Purgeable)
+        .update_secret_recovery_level(&secret_name, &secret_version, "Purgeable".into())
         .await?;
 
     // Update secret to expire in two weeks.