Added metadata column to jobs table

[ronaldlangeveld]

ref https://linear.app/tryghost/issue/ONC-11/avoid-expensive-latest-event-timestamp-queries-in-email-analytics

• added a new metadata column to the jobs table.
• as of mysql8, json data types are supported in which case we have to update the schema tests to be able to support this data type.

[github-actions]

It looks like this PR contains a migration 👀
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Schema changes

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Data changes

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