[Request]: IPv6: allow DNS configuration update after `container.create()`.

[jglogan]

Feature or enhancement request details

After the container VM starts, container.create() calls the agent to write the DNS configuration.

For user-defined configurations, or for IPv4 DNS autoconfig (using the DNS proxy on the NAT bridge), all information is known prior to creation and can be supplied in the initial configuration.

For IPv6 DNS autoconfig, it's necessary for the NAT bridge to be up and running, and for IPv6 address setup to be complete before the DNS proxy address is available. We can approach this in a couple of ways:

• Acquire the server address using SystemConfiguration dynamic store notifications (see Autoconfigure IPv6 DNS proxy. container#1007), and then update the DNS configuration using the vminitd API).
• In vminitd, monitor router advertisements for RDNSS information and apply the configuration updates.

We also need to figure out how best to apply the updates. We don't want to rely on the autoconfiguration taking place within a set time period, or at all – we don't want to block starting the workload on configuration completion.

Updating resolv.conf after the workload starts presents other challenges, though.

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