Spectral validation PoC

[willosborne]

Adds spectral validation rules to inspect implementation documents.

Rules:

• check that documents have nodes and relationships as top level elements (error, this is a very simple example rule)
• check that every relationship references a node that exists in the document (error)
• check that every node referenced by a composition relationship exists (error)
• check that every node is referenced by at least one relationship (warning)

[willosborne]

Addresses issue #20

[rocketstack-matt]

I think this going to be really useful, we should create an issue to add a GitHub Action to run the rules against every pattern we add to the repo.

We should also raise a task to review these rules every time the schema changes to ensure they're still valid.