Finish the feature for providing information about function selectors on hover

[cameel]

It is partially implemented using Dune API but the LSP was crashing when I tried to use that with QtCreator on the hackathon. See dune-request-on-hover branch.

My suggestion would be to not use Dune for this at all and switch to https://www.4byte.directory instead, which also seems to be pretty relaxed about access to the data:

The data from this service is given free of any license or restrictions on how it may be used.

Usage of the API is also granted with the single restriction that your usage should not disrupt the service itself. If you wish to scrape this data, feel free, but please do so with limited concurrency. You are encouraged to scrape your own copy of this data if your use case is likely to produce heavy load on the API.

The feature should be properly covered by tests and documented.