Exclude HostAgent implementation in release builds (#49813)

Summary:
Pull Request resolved: #49813

Making use of the recently standardised `REACT_NATIVE_DEBUGGER_ENABLED` preprocessor flag, conditionally swap out `HostAgent` for a stub in production (non dev, non profiling) builds. This uses the same private implementation idiom as `HermesRuntimeTargetDelegate`.

This results in a **42.2kiB** size reduction (uncompressed) in production builds, due to excluding the CDP agent components of `jsinspector-modern`.

- Intentionally, we're gating just this logic, rather than several outer integration points/native bindings for `jsinspector-modern`. This is for predictability and maintenance — the Fusebox subsystem is always loaded/bound, but can fork to an empty implementation.

 {F1975625351}

Changelog: [Internal]

Reviewed By: hoxyq

Differential Revision: D70495853

fbshipit-source-id: 00253ac71444819a917a3781efe38365acb91e85

Author: huntie