Add .tools.graph_viz_tool

[macflo8]

Following on the 2024-12-12 MESSAGE meeting and #580, this PR aims to add a tool that can visualise a MESSAGEix model structure as a graph.

The initial commit adds the standalone tool developed by @macflo8, which can serve as a basis for a tool that is fully integrated in the message_ix code base. The standalone tool offers two visualisation options:

• a static "RES-style" visualisation that relies on the graphviz package that is similar to the VBA tool mentioned in https://github.com/iiasa/message_ix/issues/580#issuecomment-2538468377
• an interactive graph that relies on the pyvis package that is more suited to visualise a subset of the model

How to review

• The general methodology should be reviewed to make sure the tool is computationally efficient.
• The design should be reviewed to make sure it fits with the patterns used in message_ix.
• The ease of use should be reviewed, to increase the probability that current (and future) message_ix users will actually use it.

PR checklist

• [ ] Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation.
• [ ] Update release notes.