Let users customize their web client

[irevoire]

Following: #426 (comment)

It would be nice to make the web client customizable, i.e:

• Add a feature flag to get rid of ishac (by default, we should continue to use it)
• Add a new_with_client function to create a Client with anything that implements request

[carlosb1]

I could try to do a proposal to simplify the constructor... I can try a builder pattern or factory... but I could not use the draft PR... I would need to do a new impl... Is it fine ?
Some refs:
https://rust-unofficial.github.io/patterns/patterns/creational/builder.html