Release Candidate for flexible JSON support and error codes

This RC features the introduction of jwt::basic_claim which is template around a traits object providing the implementation details for using the JSON library of your choosing. 🎉

For performance sensitive code, among other reasons, there is now the introduction of std::error_code support

auto verify = jwt::verify({}).allow_algorithm(jwt::algorithm::hs256{ "secret" });
std::error_code ec;
verify.verify(decoded_token, ec);
if(ec) {
   // handle failures
}

Special thanks to @faustocarva @matze @Sp3EdeR for their contribution helping make this RC possible ❤️

Full Changelog: v0.4.0...v0.5.0-rc.0