http server: propagate extensions in http response

[niklasad1]

Close #1509

This PR propagates the extensions set by the RPC middleware such that users can inspect them via HTTP middleware and perhaps indicate a HTTP error or something.

The handling of batch requests are not ideal because because the if the same type is used more than once then the
most recent wins in the old data is overwritten. Because the http::Extensions works based on the rust type ids and it's not possible to merge two types in a clean way I think i.e, just overwrite them with the most recent one...

I had in mind that the user could inject a Vec or another collection that each call RPC call/request can push to it but we can't do that internally in jsonrpsee without some hacks like merging the rust collection types ourselves such as Vec<String>, Vec<usize> and so on...

For now, I don't have a good solution except make user responsible to "inject unique types" because it may be overwritten in a batch but annoying....

We could introduce another representation of the extensions internally and just merge them before constructing the response though...