Add schema validation, require @entity in schema types

[Jannis]

Resolves #236. Depends on #320. Last puzzle piece in finishing #279.

This makes @entity mandatory on all type definitions in subgraph schemas. If we later want to distinguish between embeddable/inline types that are not represented as entities, we can relax this requirement again. (In that case we should e.g. verify that the inline types are actually referenced anywhere.)

Subgraph schemas are now validated (with the above being checked) when subgraphs are deployed. If a subgraph schema fails to validate, the subgraph is rejected.

Now that it's mandator, we can use the @entity directive to properly collect entity types involved in subscription queries, and only subscribe to entity change events from the store for these entities. For example, the subscription

subscription { memes { id } }

would result in a store subscription for [(<subgraph ID>, Meme)]. The subscription

subscription { memes { id regEntry_creator { id } } }

would result in a store subscription for [(<subgraph ID>, Meme), (<subgraph ID>, User)].

[Jannis]

Note: We should update the docs as part of this, to make sure people are aware of the introduction of @entity.

[leoyvens]

In bad need of a rebase.

[Jannis]

@leodasvacas Done. Care to take another look?

[leoyvens]

Looks good apart from minor comment. It'd be good to give the team a heads up and also update our adchain and decentraland subgraphs, so nobody gets a bad surprise when trying to demo something at ETHBerlin.

[leoyvens]

These turbofishes seems uncessary, also I wonder If we could just use Err and Ok instead of err and ok.

[Jannis]

You were right. Updated!

[Jannis]

@leodasvacas Totally agree. I'll get PRs ready for all public subgraphs + docs before we merge this.