Apisix-dashboard: support debug file transfer in online debug

[liuxiran]

Feature request

Please describe your feature

In the process of solving #1196, I found that the current online debug does not support debugging file transfer. It would be better to support this feature since APISIX support it.

Describe the solution you'd like

I would like to implement this feature in form of request forwarding the Content-type: multipart/form-data, just refer to postman.

[liuxiran]

a TODO feature, reopen now

[Jaycean]

@liuxiran @juzhiyuan I can help with the BE, please assign to me ,Thks.