[FEATURE] Basic GDN framework in Mava

[mmorris44]

Feature

High-level Issue for getting GDNs into Mava.

Proposal

Will use a branch (feature/gdns) that I merge the other PRs into. Will merge from feature/gdns to develop.

Definition of done

Basic GDNs working in Mava and solving Easy Traffic Junction.

Mandatory checklist before making a PR

• [ ] The success criteria laid down in “Definition of done” are met.
• [ ] Code is documented - docstrings for methods and classes, static types for arguments.
• [ ] Code is tested - unit, integration and/or functional tests are added.
• [ ] Documentation is updated - README, CONTRIBUTING, or other documentation.
• [ ] All functional tests are green.
• [ ] Link experiment/benchmarking after implementation (optional).