Add possibilty to add variables to scheduled pipelines

Problem to solve

Currently I cannot create schedule pipelines with variables in them.

Proposal

Add an option to add variables during creation of the schedule:

glab schedule create --cron "* * * * *" --description "example" --ref "main" --variable "foo=bar" --variable "baz=bat"

Further details

I'll work on this. :)

Links / references