On-chain metadata indexing

[ptrwtts]

Add the ability to get metadata for a collection on-chain, via the getTokenURI. We would not use this as the default (for scaling reasons), but we would at least have it as an option, that would be useful in certain scenarios, such as immediately after a reveal.

[dandlezzz]

I also, would like the ability to have this. Working with a non-deterministic getTokenURI function.