Return the host IPvFuture address
<include/boost/url/authority_view.hpp>
core::string_view
host_ipvfuture() const noexcept;
If the host type is host_type::ipvfuture , this function returns the address as a string. Otherwise, if the host type is not an IPvFuture address, it returns an empty string.
assert( url_view( "http://[v1fe.d:9]/index.htm" ).host_ipvfuture() == "v1fe.d:9" );
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )