[Feature]: New-AzSqlDatabaseExport the resulting bacpac file is not encrypted

### Description of the new feature

When using New-AzSqlDatabaseExport the resulting bacpac file is not encrypted. My requirement is that the bacpak file be encrypted on blob storage in case someone gains access to the blob storage or keyvault.

### Proposed implementation details (optional)

Add new encryption key parameter and use aes/pgp to encrypt the bacpak file as [filename].bacpak.encrypted

Start at the New-AzSqlDatabaseExport cmdlet implementation and trace how the bacpac is written to Blob Storage. Clarify the encryption-key parameter and AES or PGP approach before proceeding; done means the exported bacpac is encrypted at rest and the resulting filename follows the requested encrypted form.

Written by the indexing model from the issue text.