Add --proxy option to enable the network proxy for the http requests in the android emulator.

Is your feature request related to a problem? Please describe.
Currently, I couldn't set a network proxy for the http requests in the android emulator. I noticed that avd has this option but tns doesnt. And I did all the searching in the documents and stackoverflow and still didnt find one. Currently I am on a project that have to request facebook's api, but I am in China, I cant access those api with proxy in the android emulator, so I am stuck on it now.

Describe the solution you'd like
Add --proxy option to enable the network proxy for the http requests in the android emulator like avd -http-proxy did.

Describe alternatives you've considered
I think --proxy is the most straightforward solution, or u could add some additional file in the project to have this function.

Additional context
I think its a key function, for users in mainland China, without proxy or vpn, they cant access public api from Twitter or Facebook, so please add this function ASAP, thanks.

首先跟踪 Android 模拟器的启动路径和 CLI 现有的选项解析，并以所要求的 --proxy 行为和 avd -http-proxy 作为参考。完成的标准是：CLI 接受该选项，将其传递给模拟器的 HTTP 代理配置，并通过模拟器经由代理发出的请求进行验证。