Add default_desired_privilege_level option

[cknost]

Is your feature request related to a problem? Please describe.
You need the enable password in the netbox config to enter privilege_exec mode.

Describe the solution you'd like
A new parameter named default_desired_privilege_level, which define a new default desired privilege level for scrapli.

Additional context
I will make a merge request.

[miaow2]

Hi @cknost, thanks for opening the issue and PR! I will take a look as soon as I will have free time.