Add support for API keys

[dshemetov]

• adapt for required api_key #3 exists, but seems to assume that these API keys will deprecate existing auth methods, which is not the case in the initial roll out of API keys
• see feat: add auth functions and add arg to covidcast epidatr#79 for the R implementation; the Python implementation might be slightly different (e.g. use .env and the dotenv package)

[dshemetov]

Closed in f7930a9