[Feature]][Proposal] Describe Tiingo API with OpenAPI

As time has passed, various changes have been made or requested for the Tiingo API as new endpoints were added or removed which aren't using Python specific details

- #337
- #82
- #317
- Others that are sure to come in the future

In an effort to make it easier for people to work with Tiingo and other APIs as fields are added or removed over time, I've been using `stoplight.io` to write OpenAPI (formerly known as Swagger) specifications (specs).

OpenAPI specs are YAML file descriptions of how entire APIs work. From an OpenAPI spec, several libraries exist for different languages (Python, Javascript, Typescript, etc), and "client libraries" for consuming those APIs could be automatically generated. A large [ecosystem](https://openapi.tools/) of tooling exists around it.

I'm considering auto-generating half of the Tiingo API client (so all new endpoints and parameters will be automatically up to date), and grafting it onto the special features we've added to `tiingo-python` (like the Dataframes and "get supported tickers") integrations.

The first step to this is will be to describe a few of the endpoints over at https://api.tiingo.com/documentation/general/overview. I'll update this issue with subtasks or more concrete guidelines based on interest / how preliminary tests turn out.