Refactor `sqlcmd create` functionality into it's own pkg

[stuartpa]

The sqlcmd create mssql command and all of the surrounding lifecycle commands (start/stop/delete) etc. is a complete end to end scenario that would be useful as it's own golang pkg which can be imported/re-used/tested as it's own package.

We should probably also create a separate package just the functionality that operates over ~/.sqlcmd/sqlconfig