feat: add command to fetch render tree by widget type

[Alpaca00]

Hi there,

this is a client-side implementation for this PR

[saikrishna321]

@Alpaca00 Thanks for the PR. Is widgetType mandatory? Reason to ask is if we can get the entire tree then we can build an inspector.

[Alpaca00]

@saikrishna321
I made this parameter optional.
Regarding the inspector - it's a great idea, but it should offer something better than the existing Flutter Inspector.
One more nuance is that the rendering doesn't capture properties of all widgets, since some of them encapsulate those properties.

Here's an example of the render tree response from the root without any parameters for demo-app
https://gist.github.com/Alpaca00/dd56540be081af049ed8e6b6ca1893db