Add X-API-KEY for api request

Our API for dashboard is using API Key to auth the request, please add the API Key Header in global request handler. Just like [1] for reference. Please note, this key should be added by config file, such as .env file. I recommend fetching this key by fetchAPIKey API.

[1] b3b3065#diff-084c3d9c2786b7cd963be84e40a38725R32

Difficulty: Minor

mentors: juzhiyuan@apache.org
Potential mentors:
Project Devs, mail: dev (at) apisix.apache.org