make SR OS to accept startup-config in partial form

[hellt]

SR OS nodes must be able to accept startup config both in full form and in partial, CLI-blob-like format. The full form is a hardcopy of a config file which is used by the node to boot from. We store this file at tftp location from which SR OS boots.

The partial config is provided in the MD-CLI format and must conform to the following structure:

1. entering into the configuration mode
2. commands to execute
3. commit command

SR OS vrnetlab code needs to be adjusted to make an analysis if the provided file contains the full config blob, or a partial command. The way to do this is under investigation. If the full config is detected to be provided, the current startup config implementation is enforced.

If the partial config is detected, we should send the commands after the node boots, similar to how startup config is provided for XR, VMX, etc.