Enhance support for capabilities within the devtools-api

[alexzhang1030]

Hello!
Thank you for your amazing project!
I have a question.

Recently addCustomTab has been supported. Its API supports iframe, so 3rd tools gives us more freedom for debug tools.
https://devtools-next.vuejs.org/plugins/api#addcustomtab

Currently, I seems that it does not support RPC, are there any plans to do so in the future?

I know from reading the source code that vite-plugin-insect is integrated using addCustomTab.
vite-plugin-insect uses vite's RPC and negociates with its vite dev server and browser client.

I hope that support the RPC between the iframe side and on the vue devtools side.

Originally posted by @kazupon in #112 (comment)