Added custom headers support for Aria2 RPC HTTP(s) requests

[JerryLocke]

This PR will add a "Aria2 RPC Http Request Header" option in AriaNg settings, which allows users to customize the headers of each RPC HTTP request.

Some users use the Cloudflare Access service to block others from illegally accessing their NAS at home, thereby improving the security of the home network. That service usually requires users to log in with OAuth for authentication. For automation or CI programs, users can also add specific headers(CF-Access-Client-Id and CF-Access-Client-Secret) to the http request to pass the verification of Cloudflare.

So this PR will benefit users similar to the above scenarios.