Support for API key

[pmcelhaney]

Counterfact doesn't currently read the security object of the OpenAPI schema.

When generating types, it should take that object into account.

For example, if the security object is defined as:

{
  "type": "apiKey",
  "name": "api_key",
  "in": "header"
}

Then Counterfact should act as if every operation has a required request header named "api_key" when generating code.

Note: this only covers "apiKey" authentication. OpenAPI also "http", "mutualTLS", "oauth2", "openIdConnect". Eventually we'll want to support "http" and "oauth2" as well. I'm not sure the others worth pursuing.