[Feature]: Get-AzRedisCacheKey should use securestring to protect key in result

### Description of the new feature

Below is current return:
```
Get-AzRedisCacheKey -ResourceGroupName "MyResourceGroup" -Name "MyCacheKey"

PrimaryKey : pJ+**********=
SecondaryKey : pJ+**********=
```

data type of PrimaryKey and SecondaryKey should be securestring.

### Proposed implementation details (optional)

_No response_

Start from the Get-AzRedisCacheKey cmdlet and trace how its PrimaryKey and SecondaryKey result properties are defined and returned. Check the existing cmdlet tests, if present, for the result shape and add coverage for the requested types. Done means both properties are returned as SecureString values while the cmdlet continues to retrieve Redis cache keys correctly.

Written by the indexing model from the issue text.