Add generic objects to the topology

[guicassolato]

Refactoring of the Topology struct (mainly) to better support generic objects added to the graph.

In general, allows creating and navigating topologies such as the following:

• Changes the topology functions so the graph can be searched and navigated for generic objects:
  • Listing functions Targetables(filters...) and Policies(filters...) have been modified to now return pointers to collections of that type, that can be navigated through the graph methods Roots(), Parents(), Children(), and Paths()
  • Navigation can be inflected from one type to another – e.g. topology.Objects().Children(gateway) to return all generic objects that are children of the gateway (targetable)
• Make policy kinds generically linkable – useful to associate them to generic objects
• Changes the shape, style and colors used to represent objects in the topology in Graphviz DOT language – now:
  • Targetables → colored square boxes
  • Policies → white dashed notes
  • Other objects → white solid ellipses
• Add/export controller types/functions (useful for implementing reconcilers)
  • Object struct (previously genericObject) – for easy wrapping of any runtime object kind
  • New function Destruct - to create and and update objects from concrete types
  • Restructure function – for consistency with Destruct (also public)
  • Cache Store – so access to the objects can be given to build runtime link functions
• Adds new examples of the Kuadrant custom controller working alongside with Envoy Gateway and Istio (to demo reconciliation of other generic objects appended to the topology)

[KevFan]

Verified envoy-gateway.md and multiple-gateway-providers.md and both are working as expected.

Looks good to me! 🎖️