Change hardcoded exchanges to be dynamically added

[davekaj]

Good timing, apparently the issue that was blocking us from doing dynamic data sources is fixed graphprotocol/graph-node#892 .

I haven't looked into it deeply, but from what I interpret, we can now get null returned instead of the graph node crashing. This should allow us to handle all contracts dynamically. This will also allow us to make token, decimals, and name all required in schema.graphql (Noah did this in his PR but I just noticed we didn't add it in Ians PR)

@ianlapham take a look at this, let me know if you have any questions. If you can implement it that would be amazing! This week I am busy but I can get to it next week. If you need help reach out on discord.

[ianlapham]

This is perfect - I'm going to make another PR that has these changes. Thanks for the help digging into all of this.