Improved Middleware

[ItzNotABug]

Current middleware behaviour only allows to add a interceptor for incoming requests. This PR adds support to intercept the outgoing responses too.

The middleware behaviour is compatible with the previous implementation.

Example - Remove cookies from request & response.

appExpress.middleware({
  incoming: (req) => delete req.headers.cookie,
  outgoing: (_, interceptor) => delete interceptor.headers.cookie,
});

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The interceptor contains the processed body which could be a String or a Buffer, statusCode & the headers.