Add on_success and on_failure command hooks

[WillDaSilva]

@longtomjr suggested the following in the original issue for cron-ext:

I don't know if it will be out of scope for this feature request, but it might be valuable to have an option to integrate with cron / dead man's switch tools like https://healthchecks.io/.

It might also be valuable for setups based on the proposal at #5962.

The simplest way I can think of would be to add a way to have an option for an extra command that will be be added to the cron entry.

So instead of generating

0 0 * * * (cd /project && meltano run ...) 2>&1 | /usr/bin/logger -t meltano

meltano generates the following cron

0 0 * * * (cd /project && meltano run ...) 2>&1 | /usr/bin/logger -t meltano && <user defined command here>

It should be possible to specify these on_success/on_failure commands in meltano.yml as a setting for the cron utility (accessible to cron-ext as an environment variable), or by providing them with command-line flags.

A list of commands can be provided to be run in order on success/failure as appropriate.

Non-zero exit codes from the on_success/on_failure commands will be ignored.