Add a middleware for browser refresh. (#24574)

* Add a middleware for browser refresh.

* Introduce a middleware that can connect to the dotnet-watch change server
* dotnet-watch: Inject the middleware in 3.1 or apps using start hooks \ hosting startup

https://github.com/dotnet/aspnetcore/issues/23412

* Update src/Tools/dotnet-watch/BrowserRefresh/src/StartupHook.cs

* Changes per PR comments

* Add a test for reading the script

* Changes per PR comments

* Updates docs

* Fixup test

* Add project ref