Add static DNS server migration

[jcronenberg]

Previous discussion revealed that probably a full migration isn't possible (e.g. NETCONFIG_DNS_POLICY="STATIC_FALLBACK" isn't possible with NetworkManager).
But e.g. NETCONFIG_DNS_STATIC_SERVERS should be migrated.

[jcronenberg]

As for the implementation, AFAICT Netconfig isn't represented inside wicked show-config, so we'll need to implement a way to read /etc/sysconfig/config via e.g. https://github.com/dotenv-rs/dotenv

[jcronenberg]

The way I see it there are basically 2 possibilities to 'imitate' the netconfig behavior for NM.
A global DNS server configuration or adding the DNS servers to every connection in the ipv*.dns property.
Here's what I think some of the advantages and disadvantages for these are:

Global DNS

Pros:

• The closest to the netconfig behavior
• Maps better to what is actually written in /etc/resolv.conf
• IMO better for servers where a lot of connections are more common and changing them globally simultaneously is more important

Cons:

• Requires a configuration file either in /etc/NetworkManager/conf.d or /var/lib/NetworkManager/NetworkManager-intern.conf
• Due to configuration file changes on a running system this requires a NM restart to take effect
• This setting isn't accessible by nmcli only via DBUS

Setting DNS for every connection

Pros:

• IMO seems to be the NM 'way' at least from what I read online, so what users may be used to.
• IMO better for clients/simple setups where single connections are more common and changing quickly via nmcli is more important

Cons:

• Tedious to change DNS servers for all connections
• If single connections are changed it is not transparent what gets actually written to /etc/resolv.conf. It seems NM chooses DNS servers based on some priority setting, which we won't set for the migration. So when a single connection is changed I personally don't know what actually happens.

For #75 I chose the global DNS config approach because I personally think it's the better option but changing it to the other approach can be done relatively quickly, so it's no problem if we choose differently in the end.

[cfconrad]

Due to configuration file changes on a running system this requires a NM restart to take effect

It looks like there is a way to reread only the dns-configuration https://gitlab.freedesktop.org/NetworkManager/NetworkManager/blob/main/src/nmcli/general.c#L350

[cfconrad]

This enum somehow explain quite good what NetworkManager does: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/blob/main/src/core/dns/nm-dns-manager.h#L101