Consider Rhai as a scripting environment

[github2023spring]

Supporting rhai script to modify the request body, and response body like the AWS API Gateway with Velocity Template

[jamesmunns]

Hi there! We do plan to add a scripting environment after the 1.0 release, though the current plan is to investigate WASM as the scripting environment, as opposed to Rhai.

This is subject to change though!

[github2023spring]

Sounds great! Thank you!