Add ENABLE_NETWORK_THROTTLING experiment

[huntie]

Summary

Adds an experiment enabling the Network throttling UI when configured by a supported React Native target.

🚧 This is a development phase feature which we may not ship, and is controlled by a corresponding native feature flag in the React Native codebase.

Test plan

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.