extensions: introduce another data structure that support "appending" instead of "replacing"

[niklasad1]

jsonrpsee relies on http::Extensions both for http middleware and rpc middleware which works based on Rust type ids so if one injects Vec<String> in the extensions for instance in a batch request with several calls the last item "wins" and simply replaces it.

This is not ideal and would be much better to use some other internal data structure to merge such data

There is another crate https://docs.rs/polymap/latest/polymap/polymap/struct.PolyMap.html that may be useful for this

For further context see #1509 and #1514