OpenAPI basic functionality

[ivnsch]

Generate basic (no custom types) code and integrate into our current file structure.

Specs:
algod: https://github.com/algorand/go-algorand/blob/master/daemon/algod/api/algod.oas3.yml
indexer: https://github.com/algorand/indexer/blob/develop/api/indexer.oas3.yml

Could be submitted as a feature branch, to be continued in #165 (possibly by a different contributor)