Support config_path for cron expressions

[peterjaap]

While working on #97, I discovered that this extension does not show the frequency for cron jobs that use a config_path field instead of a schedule field.

For example the currency_rates_update job;

It's crontab.xml looks like this;

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
    <group id="default">
        <job name="currency_rates_update" instance="Magento\Directory\Model\Observer" method="scheduledUpdateCurrencyRates">
            <config_path>crontab/default/jobs/currency_rates_update/schedule/cron_expr</config_path>
        </job>
    </group>
</config>

[peterjaap]

Added here; #203

You can test this by setting the currency_rates_update cron expression through magerun2;

magerun2 config:store:set crontab/default/jobs/currency_rates_update/schedule/cron_expr "0 * * * *"
magerun2 cache:clear config

[fredden]

This has been completed in #203, which landed in v2.1.6.