Allow storing detection method in Vulnerabilities::Finding

What does this MR do?

When working on #10272 (closed) it came to our attention that we don't have any way of storing the detection method used to discover vulnerability findings. This MR adds that capability to the underlying data model

Related: #332473 (closed)

Migrations

UP

== 20210614131002 AddDetectionMethodToVulnerabilitiesFinding: migrating =======
-- add_column(:vulnerability_occurrences, :detection_method, :smallint, {:null=>false, :default=>0})
   -> 0.0021s
== 20210614131002 AddDetectionMethodToVulnerabilitiesFinding: migrated (0.0022s)

DOWN

== 20210614131002 AddDetectionMethodToVulnerabilitiesFinding: reverting =======
-- remove_column(:vulnerability_occurrences, :detection_method, :smallint, {:null=>false, :default=>0})
   -> 0.0031s
== 20210614131002 AddDetectionMethodToVulnerabilitiesFinding: reverted (0.0058s)

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