[KeyVault] Fixed CertificateString decoding issue in Import-AzKeyVaultCertificate (#20172)

* Fixed CertificateString decoding issue in Import-AzKeyVaultCertificate

* add change log

* update MD

Author: BethanyZhou

src/KeyVault/KeyVault/ChangeLog.md

@@ -18,6 +18,7 @@
         - Additional information about change #1
 -->
 ## Upcoming Release
+* Fixed CertificateString decoding issue in `Import-AzKeyVaultCertificate`
 * Shifted the location of key CVM release policy to GitHub [#19984]
 * Added fallback logic (reading default CVM policy from a local copy) if fetching default CVM Policy from GitHub failed.
 

src/KeyVault/KeyVault/Track2Models/Track2KeyVaultDataServiceClient.cs

@@ -241,7 +241,7 @@ public IEnumerable<PSDeletedKeyVaultCertificateIdentityItem> GetDeletedCertifica
 
         public PSKeyVaultCertificate ImportCertificate(string vaultName, string certName, string certificate, SecureString certPassword, IDictionary<string, string> tags, string contentType = Constants.Pkcs12ContentType)
         {
-            return VaultClient.ImportCertificate(vaultName, certName, Encoding.ASCII.GetBytes(certificate), certPassword, tags, contentType);
+            return VaultClient.ImportCertificate(vaultName, certName, Convert.FromBase64String(certificate), certPassword, tags, contentType);
         }
 
         public PSKeyVaultCertificate ImportCertificate(string vaultName, string certName, byte[] certificate, SecureString certPassword, IDictionary<string, string> tags, string contentType = Constants.Pkcs12ContentType)

src/KeyVault/KeyVault/help/Import-AzKeyVaultCertificate.md

@@ -23,8 +23,8 @@ Import-AzKeyVaultCertificate [-VaultName] <String> [-Name] <String> -FilePath <S
 ### ImportWithPrivateKeyFromString
 ```
 Import-AzKeyVaultCertificate [-VaultName] <String> [-Name] <String> -CertificateString <String>
- [-Password <SecureString>] [-Tag <Hashtable>] [-DefaultProfile <IAzureContextContainer>] [-WhatIf] [-Confirm]
- [<CommonParameters>]
+ [-ContentType <String>] [-Password <SecureString>] [-Tag <Hashtable>]
+ [-DefaultProfile <IAzureContextContainer>] [-WhatIf] [-Confirm] [<CommonParameters>]
 ```
 
 ### ImportWithPrivateKeyFromCollection
@@ -151,6 +151,21 @@ Accept pipeline input: False
 Accept wildcard characters: False
 ```
 
+### -ContentType
+Specifies the type of the certificate to be imported. Regards certificate string as PFX format by default.
+
+```yaml
+Type: System.String
+Parameter Sets: ImportWithPrivateKeyFromString
+Aliases:
+
+Required: False
+Position: Named
+Default value: None
+Accept pipeline input: False
+Accept wildcard characters: False
+```
+
 ### -DefaultProfile
 The credentials, account, tenant, and subscription used for communication with azure