Add retry mechanics to `pallet-scheduler` (#3060)

Fixes #3014 

This PR adds retry mechanics to `pallet-scheduler`, as described in the
issue above.

Users can now set a retry configuration for a task so that, in case its
scheduled run fails, it will be retried after a number of blocks, for a
specified number of times or until it succeeds.

If a retried task runs successfully before running out of retries, its
remaining retry counter will be reset to the initial value. If a retried
task runs out of retries, it will be removed from the schedule.

Tasks which need to be scheduled for a retry are still subject to weight
metering and agenda space, same as a regular task. Periodic tasks will
have their periodic schedule put on hold while the task is retrying.

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Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: command-bot <>