Draft: POC - extend zoekt with knowledge graph

What does this MR do and why?

POC which adds knowledge graph models for deploying knowledge graph service on zoekt nodes.

Related to #534843 (closed) and gitlab-com/content-sites/handbook!13104 (merged)

The scope of this POC is limited to to just defining models necessary for knowledge graph and creating a service which schedules indexing of a graph DB on repository replicas.

Complementary zoekt-indexer MR is needed for trying this POC - gitlab-zoekt-indexer!472 (closed)

The schema of models looks like this:

classDiagram
    namespace ZoektModels {
    class Node
    class Index
    class Repository
    class Task
    class EnabledNamespace
    class Replica
    }
    namespace KnowledgeGraphModels {
    class KnowledgeGraphEnabledRepository
    class KnowledgeGraphReplica
    class KnowledgeGraphTask
    }

    Node "1" --> "*" Task : has_many tasks
    Node "1" --> "*" Index : has_many indices

    EnabledNamespace "1" --> "*" Replica : has_many replicas

    Replica "1" --> "*" Index : has_many indices

    Index "1" --> "*" Repository : has_many repositories
    Repository "1" --> "*" Task : has_many tasks

    Node "1" --> "*" KnowledgeGraphTask : has_many graph tasks
    KnowledgeGraphEnabledRepository "1" --> "*" KnowledgeGraphReplica : has_many replicas
    KnowledgeGraphReplica "1" --> "*" KnowledgeGraphTask : has_many tasks
    Node "1" --> "*" KnowledgeGraphReplica : has_many graph replicas

This allows keeping Zoekt and Knowledge graph models separate, except re-use of Zoekt node.

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