[vnet][3] DNS resolution

[nklaassen]

This is the fourth in a series of PRs implementing Teleport VNet RFD. parent child

This PR adds DNS resolution for Teleport apps in any currently active Teleport profile. Each app is assigned a V6 IP address the first time a DNS A or AAAA record is queried for its public_addr. Since IPv4 isn't supported yet, we return a NoRecord response for A queries. Clients should use the v6 address they can get from an AAAA query.

TODO: Host configuration of the DNS nameserver is added in https://github.com/gravitational/teleport/pull/41032 TODO: TCP forwarding to the app works in the mocked testcase, but forwarding to real Teleport apps (along with app logins) is added in https://github.com/gravitational/teleport/pull/41033 TODO: Client caching is added in https://github.com/gravitational/teleport/pull/41033 TODO: custom DNS zones and leaf cluster support (coming in a future PR)

[nklaassen]

friendly ping @ibeckermayer or @EdwardDowling I'm looking for one more review on this one

[nklaassen]

@ibeckermayer this is ready for another look