How to custom the executor thread count in v2

[Alps2]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
For example, replace the 4 thread count by writing a new FLUTTER_RUST_BRIDGE_HANDLER in v1

Describe alternatives you've considered
Can you please provide a parameter to assign the default thread count of executor.

Additional context

[fzyzcjy]

Just customize your handler https://cjycode.com/flutter_rust_bridge/guides/custom/rust#customize-handler

This is the default code:

flutter_rust_bridge/frb_rust/src/for_generated/boilerplate.rs

Line 136 in 345c3f5

 macro_rules! frb_generated_default_handler { 

And you can copy-paste-modify it in your project. If frb detects the FLUTTER_RUST_BRIDGE_HANDLER variable, it will stop using the default template automatically.