Implement bearer token auth

[Siegrift]

Closes #22

Rationale

The Signed API configuration changes are inspired by the following idea:

{
  "endpoints": [
    {
      "urlPath": "/default",
    -> "authTokens": ["token"],
      "delaySeconds": 0
    },
    {
      "urlPath": "/delayed",
   -> "authTokens": null,
      "delaySeconds": 15
    }
  ],
  "allowedAirnodes": [
    {
      "address": "0xbF3137b0a7574563a23a8fC8badC6537F98197CC",
   -> "authTokens": ["airnode-token-1", "airnode-token-2"]
    }
  ],
  "stage": "local",
  "version": "0.1.0"
}

Each endpoint can have Bearer token authentication. Similarly for the endpoint to push the data from Airnode feed. Read the configuration changes (READMEs) and the changes in the example configuration files.

Related

• Readme config changes
• Update spec (high level reasoning why we use auth)