Add --description-from-stdin option to snapshot creation

[jandubois]

On Windows the commandline has a maximum length of 32KB, which limits the amount of data you can pass to the --description option (see #5843).

An easy way around this would be to add an alternative --description-from-stdin option, so the app could pipe the description to rdctl.

It is arguable that 32KB should be plenty enough for a snapshot description, but it is possible that the description may be autogenerated from manifests of all the workloads installed inside the VM, so it is better to not have an artificial limit.

[ericpromislow]

I would add support for --description-from-file FILE because stdin on powershell isn't so simple, so some users might not be comfortable with it.

[jandubois]

I would add support for --description-from-file FILE

I'm fine with this as long as --description-from file - works as well, to implement --description-from-stdin.