OpenAPI spec

**Is your feature request related to a problem? Please describe.**
Hey, was wondering if there is an available OpenAPI spec for the Alpaca API. Otherwise, devs are sort of left to only use the official clients, which is limiting in the languages that will be available.

**Describe the solution you'd like**
An OpenAPI spec for teams to leverage code generation in their chosen language.

**Describe alternatives you've considered**
Using the official SDKs

**Additional context**
N/A