Cron scheduler

[andrzej-k]

Snap shall support a new type of scheduler - cron scheduler - which will allow users to use cron-like syntax to define task interval. For example:

{
    "version": 1,
    "schedule": {
        "type": "cron",
        "interval" : "@every 3s"
    },
    "workflow": {

or

{
    "version": 1,
    "schedule": {
        "type": "cron",
        "interval" : "0 30 * * * *"
    },
    "workflow": {

also

snapctl task create --workflow-manifest --interval

shall also support passing cron-style interval