Consider adding Bisections as part of the data model

I haven't seen bisections mentioned in the code or the documentation so I don't know where they're supposed to fit in the final design.

In the legacy KernelCI, bisections are handled completely separately and away from the main data and we've seen the disadvantages of that, as it's not always trivial to link a bisection to the related regression/s, and it'll be really helpful to have this bi-directional link available at all times to know which regressions a bisection points to and whether a specific regression has been bisected.

No file, test, or entry point is named. Start by reviewing the existing data model and documentation to determine where bisections and their links would fit; done requires an agreed design for representing the bidirectional relationship with regressions.

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