az sql db import/export should support access token

**Describe the solution you'd like**

Currently `az sql db export` and `az sql db import` demand user and password from a database user. It should also support Active Directory Integrated authentication with an access token.

**Describe alternatives you've considered**

Continue using user and password, which is less secure.

**Additional context**

This is currently supported [in sqlcmd with PowerShell and `Invoke-Sqlcmd`](https://docs.microsoft.com/en-us/powershell/module/sqlserver/invoke-sqlcmd?view=sqlserver-ps#example-11--connect-to-azure-sql-database--or-managed-instance--using-an-access-token), and from several Sql Server drivers (see the [example for .NET](https://docs.microsoft.com/en-us/sql/connect/ado-net/sql/azure-active-directory-authentication?view=sql-server-ver15#support-for-a-custom-sql-authentication-provider)).