Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39045
## Context
RFC: Decoupling Flipper from React Native core: https://github.com/react-native-community/discussions-and-proposals/pull/641
## Changes
- Relocates `metro-inspector-proxy` source from the Metro repo into the React Native repo as part of the `react-native/dev-middleware` package.
- Drops the `runInspectorProxy` entry point.
- Attaches the Inspector Proxy to the `createDevMiddleware()` API as the new integration point for this functionality.
- Documents migrated endpoints + usage of `createDevMiddleware()` in README.
Changelog: [Internal]
Metro changelog: None (`metro-inspector-proxy` is now an internal component of `react-native`, covered in the [release notes for 0.78.1](https://github.com/facebook/metro/releases/tag/v0.78.1))
Reviewed By: motiz88, blakef
Differential Revision: D48066213
fbshipit-source-id: 3fbef5d881f6f451cb5955dcbbc362c53347437e
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