Add serverBaseUrl option, set client-accessible URL value externally (#39456)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39456

**Fixes new debugger launch flow on Android:**

D49158227 aimed to improve proxy-safe behaviour for remote dev servers by auto-detecting the appropriate server URL for clients using the `Host` header (etc) from the HTTP request. However, this approach broke the local case for Android emulators and externally connected devices since they would originate from a device-relative server hostname — e.g. `10.0.2.2` for the stock Android emulator.

https://pxl.cl/3mVmR

This commit reverts to an explicit approach where callers specify the base URL to the dev server that should be addressible from the development machine — now as a single `serverBaseUrl` option.

**Changes**

- Adds new `serverBaseUrl` option to `createDevMiddleware`, designed to be the base URL value for constructing dev server URLs returned in endpoints such as `/json/list`.
    - This changes little for the `localhost` case (now enabling `https://` URLs), but enables remote dev server setups to specify this cleanly.
- Updates call site in `community-cli-plugin`.

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D49276125

fbshipit-source-id: 2b6a8507073649832993971aa9d0870f54c9bd44