[expo] support DevToolsPluginClient websocket multiplexing on expo web (

#27702)

# Why

if an expo web app contain multiple devtool plugins, we will create
multiple websocket connection on web.

# How

- move the connection multiplexer from `DevToolsPluginClientImplApp` to
base `DevToolsPluginClient`
- the multiplexer relies on static variables. however, the unit tests
are running in single process. this pr introduces a
`WebSocketBackingStore` for test to specify their backing store. for
normal use case, the default backing store is a static variable.

packages/expo/CHANGELOG.md

@@ -13,6 +13,7 @@
 ### 🐛 Bug fixes
 
 - Fixed breaking changes from React-Native 0.74. ([#26357](https://github.com/expo/expo/pull/26357) by [@kudo](https://github.com/kudo))
+- Fixed multiple WebSocket connections created on Web when an app containing multiple dev tools plugins. ([#27702](https://github.com/expo/expo/pull/27702) by [@kudo](https://github.com/kudo))
 
 ### 💡 Others