[Built-in Rule] SQL managed instances should use customer-managed keys to encrypt data at rest

Azure Policy link: https://github.com/Azure/azure-policy/blob/master/built-in-policies/policyDefinitions/SQL/SqlManagedInstance_EnsureServerTDEisEncryptedWithYourOwnKey_Audit.json

``` js
{
"name": "Sql_ManagedInstance_EnsureServerTDEisEncryptedWithYourOwnKey",
"description": "SQL managed instances should use customer-managed keys to encrypt data at rest",
"recommendation": "Implement Transparent Data Encryption (TDE) with your own key",
"helpUri": "https://github.com/Azure/template-analyzer/docs/built-in-bpa-rules.md/#sql-managed-instances-should-use-customer-managed-keys-to-encrypt-data-at-rest",
"evaluation": {
"where": {
"resourceType": "Microsoft.Sql/managedInstances",
"path": "name",
"hasValue": true
},
"evaluate": {
"resourceType": "Microsoft.Sql/managedInstances/encryptionProtector",
"anyOf": [
{
"path": "properties.serverKeyType",
"notEquals": "AzureKeyVault"
},
{
"path": "properties.uri",
"equals": ""
},
{
"field": "properties.uri",
"exists": false
}
]
}
}
}
```