Support network specific indexing rules

[fordN]

Part of the epic of preparing for and better supporting a multi-chain world. As we move to multi-chain large scale indexers may want to create global-like indexing rules that apply to subgraphs for a specific network.

For the implementation, I have yet to write up a specific plan, but I aim to include network type rules as a new type of the existing group IdentifierType.

[fordN]

I believe this is no longer applicable now that we have multi-network support. What do you think @tilacog ?

[tilacog]

Agreed. I think #668 addresses this.