Unify DoH URI making throughout the codebase

This merge request adds a new function isc_nm_http_makeuri() which is supposed to unify how DoH URI making is done throughout the codebase.

It handles IPv6 addresses, hostnames and IPv6 addresses given as hostnames properly and replaces the similar ad-hoc code in the codebase.

The merge request also adds IPv6 support to the doth system test.

Closes #2860 (closed), #2861 (closed)