Add `finding_id` foreign key to `vulnerabilities`

What does this MR do and why?

There is an existing has_many/belongs_to association between the vulnerabilities and vulnerability_occurrences tables.

We want to make this a legit 1-to-1 relationship eventually. As a step towards that we are adding an id column that will eventually be used for a foreign key constraint.

after adding the column and updating the app code to write to the column, we need to backfill the new column. MR 130058 will handle the backfill

resolves: #418970 (closed)

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Changelog: changed EE: true

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Related to #418970 (closed)