Copy package to ConanCenter

[alexhenrie]

@offa Would you be willing to submit this package to ConanCenter to make it available to a wider audience? The instructions for submitting a new package are at https://github.com/conan-io/conan-center-index/blob/master/docs/adding_packages/README.md.

[offa]

restclient-cpp is inactive for years now unfortunately and I'm more or less maintaining the recipe for existing users (not using it myself anymore). If someone is interested in submitting, I'm glad to retire this one. :-)