feature(dev-middleware): use userAgent query parameter as fallback when header is unset (#43364)

Summary:
At Expo, we use [Expo Tools](https://github.com/expo/vscode-expo/blob/main/src/expoDebuggers.ts) to connect the [built-in vscode-js-debug](https://github.com/microsoft/vscode-js-debug) to Hermes.

Since there are a few differences in vscode vs chrome devtools, we need to enable a couple of modifications through the [`customMessageHandler` API](https://github.com/facebook/react-native/pull/43291). Unfortunately, vscode itself doesn't set the `user-agent` header when connecting to the inspector proxy. Becuase of that, we'd need a fallback to "manually" mark the debugger as being vscode ([we use this query parameter here](https://github.com/expo/vscode-expo/blob/main/src/expoDebuggers.ts#L208)).

This PR supports setting the `user-agent` through `?userAgent=` when the header is not set.

## Changelog:

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[GENERAL] [ADDED] - Fallback to query parameter based `user-agent` when header is unset

Pull Request resolved: https://github.com/facebook/react-native/pull/43364

Test Plan:
- Install [Expo Tools](https://marketplace.visualstudio.com/items?itemName=expo.vscode-expo-tools)
- Start Metro with this change.
- Connect a device.
- Run the vscode command `"Expo: Debug Expo app ..."`
- Debugger should connect, and have it's user-agent marked as:
    `vscode/1.87.0 vscode-expo-tools/1.3.0`

Reviewed By: huntie

Differential Revision: D54804556

Pulled By: motiz88

fbshipit-source-id: 1ff558ba5350811ad042d08a713438e046759feb

packages/dev-middleware/src/inspector-proxy/InspectorProxy.js

@@ -258,7 +258,7 @@ export default class InspectorProxy implements InspectorProxyQueries {
         }
 
         device.handleDebuggerConnection(socket, pageId, {
-          userAgent: req.headers['user-agent'] ?? null,
+          userAgent: req.headers['user-agent'] ?? query.userAgent ?? null,
         });
       } catch (e) {
         console.error(e);