Redo DNS configuration (#2034)

this commit changes and streamlines the dns_config into a new
key, dns. It removes a combination of outdates and incompatible
configuration options that made it easy to confuse what headscale
could and could not do, or what to expect from ones configuration.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

.github/workflows/test-integration.yaml

@@ -37,6 +37,8 @@ jobs:
           - TestNodeRenameCommand
           - TestNodeMoveCommand
           - TestPolicyCommand
+          - TestResolveMagicDNS
+          - TestValidateResolvConf
           - TestDERPServerScenario
           - TestPingAllByIP
           - TestPingAllByIPPublicDERP
@@ -45,7 +47,6 @@ jobs:
           - TestEphemeral2006DeletedTooQuickly
           - TestPingAllByHostname
           - TestTaildrop
-          - TestResolveMagicDNS
           - TestExpireNode
           - TestNodeOnlineStatus
           - TestPingAllByIPManyUpDown