feat: assign weights in graph

[miparnisari]

Description

This is the scaffolding code required to compute weights for each node and edge. It's not meant to support every kind of model - more followup PRs will come. (For example, models with cycles are not handled).

Please review each model in the weighted_graph_builder_test.go and visualize the output in https://dreampuf.github.io/GraphvizOnline/. This will help you review the correctness of the weight assignment algorithm.

Example

[elbuo8]

Why not use gonum's weighted graph? 🤔

[miparnisari]

Why not use gonum's weighted graph? 🤔

@elbuo8 several reasons:

1. gonum's weighted graph allows natively assigning weights to edges only
2. those weights are float64 only: https://pkg.go.dev/gonum.org/v1/gonum/graph#WeightedLine

[senojj]

Non-Blocking

The relationship between the weighted graph builder and the weighted graph is a bit odd. Currently the weighted graph has a dependency of its builder, which seems backwards.

Instead of coupling it all together with the current approach:

weightedGraph, err := NewWeightedAuthorizationModelGraph(model)

I would suggest decoupling them explicitly:

weightedGraphBuilder := NewWeightedAuthorizationModelGraphBuilder()
weightedGraph, err := weightedGraphBuilder.Build(model)

I know this is introducing an additional step/line of code, however I really don't see the point of having the builder, otherwise.