feat: add auth functions and add arg to covidcast

[dshemetov]

An implementation of API keys:

• API key getter function
• API key setting recipes in README.md
• add auth arg to covidcast call params
• needs some testing

[dshemetov]

Our API supports auth via three approaches:

1. requests parameter api_key,
2. HTTP Basic authentication: pass a header with "Authentication: Basic [user:pass]" (in our case user is "epidata" and pass is api key)
3. HTTP Bearer authentication: pass a header with "Authentication: Bearer [pass]" (pass is api key)

I won't implement option 1, so we don't pollute the user's request URLs and reduce chance of them sharing their API key. The latter two seem equivalent, so I'll implement the (2), since that's implicit in httr::authenticate.

[brookslogan]

Looks nice! Have a bunch of little questions/suggestions. Main things:

• add mention of query complexity limit
• typo & tweaks in warning message
• testing and CI stuff

[brookslogan]

question: Are the old-epidata-endpoint auth token arguments clearly different from the new api key? We might need to go through and say something like "an {endpoint name}-specific authentication token (not the same thing as the newer, system-wide API key)".

[dshemetov]

Yes, they are different. Agreed.