Make net::DnsQuery a movable type.

The user-supplied copy constructor / copy assignment operator cause
the implicit move constructor / move assignment operator to be deleted.
Thus, a silent copy happens when putting a DnsQuery rvalue into a
std::optional<DnsQuery> e.g. at dns_response.cc:411. This copy involves
a needless clone of a buffer.

Change-Id: Ie61216d09fb4a873b922aedfb5c8d580b5595f38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5995150
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378679}