Add the ability to connect Fastify as a custom dev server

[sowtame]

here the main problems is how to connect dev middlewares and websocket in fastify for hmr

rspack-contrib/rspack-examples#50

[9aoy]

@sowtame I add an example for it. In fastify, you can get http server instance via fastify.server

https://github.com/rspack-contrib/rspack-examples/pull/54/files#diff-b75774b55d50e8e7e1b7443569feb1184f9d229a553e5f321e02dd8a8434ec53

[sowtame]

thank you, but is there a way that the server does not restart during hmr? (can reproduce in your example)
if you use express, there is no such thing