Add support for NEDnsProxyProvider on iOS/macOS

[ameshkov]

Issue Details

Apple provides a rather flexible way for filtering DNS queries: https://developer.apple.com/documentation/NetworkExtension/dns-proxy-provider

It has several advantages over NEDNSSettings:

• It allows filtering DNS queries and applying local rules to them
• It allows forcefully fully delegating DNS traffic to DnsProxy extension which on supervised devices (NEDnsSettings can be disabled by the user, not cool if it's a supervised school device)

Proposed solution

Provide NEDnsProxy implementation that can be used by iOS and macOS apps.

Alternative solution

No response