Create job source tables

What does this MR do and why?

Adds DB migrations for the p_ci_build_sources table, which associates builds with a source. This source is stored as an enum value; at the moment, there is only one: scan_execution_policy. Associations in Ci::Build will be added in a forthcoming MR.

Using !146555 (merged) as example - Ci::BuildName is similar in how it associates a single value to a build, though in this case the value is a smallint rather than an indexed string.

Related to #458803 (closed)