Add a command to create/add a user

It would be create if sqlcmd could support the creation of database users/logins. For example, something like

sqlcmd user add -u Davide -p <Password>

for adding a SQL user into the database pointed at by the active context, or

sqlcmd user add -u damauri@microsoft

for adding a AAD user

In both cases the users would be added to the database (creating the login if necessary) and then it would also be stored in the context config so that it could be used when generating the connection string:

sqlcmd config connection-strings -u damauri@microsoft

Start by tracing the existing sqlcmd command structure, active-context handling, and the config connection-strings command. Define how user add should handle SQL users, logins, and AAD users, then verify that both user types are stored in context config and usable when generating connection strings.