http client: add max_concurrent_requests

[niklasad1]

I decided to add back max_concurrent_requests to provide a uniform API because the ws-client already provides it.
This is only opt-in because it may be annoying with the extra sempahore overhead which some use-cases may not want..

Superseeds #1470

[jsdw]

I'd generally lean towards saying "if this can be implemented outside of the library then it should be" but for the sake of consistency I don't mind it :)

[niklasad1]

I'd generally lean towards saying "if this can be implemented outside of the library then it should be" but for the sake of consistency I don't mind it :)

Yes, that was the intention by removing it but I prefer to keep the APIs somewhat consistent instead after thinking about it