[Request]: Support registering network plugins at runtime (no code changes)

[svelez]

Feature or enhancement request details

I am currently having issues with establishing a connection from inside a container to resources with my company's software-defined perimeter. My experience is similar to

• [Bug]: Network is unreachable after turning on VPN #1307
• [Bug]: Hosts behind OpenVPN (Tunnelblick) not reachable from container #1519

However, it seems to be more difficult to address since we run in a split-tunnel configuration.
While I wait for progress to be made on the above issues, I discovered that it would be possible to implement a quick solution with the help of pf rules and a network plugin. When adding support for my experimental plugin here:

container/Sources/Plugins/RuntimeLinux/RuntimeLinuxHelper+Start.swift

Lines 66 to 72 in e1a5cfd

	// FIXME: The network plugins that the runtime supports should be configurable elsewhere
	var interfaceStrategies: [NetworkInterfaceKey: InterfaceStrategy] = [
	NetworkInterfaceKey(plugin: "container-network-vmnet", variant: "allocationOnly"): IsolatedInterfaceStrategy()
	]
	if #available(macOS 26, *) {
	interfaceStrategies[NetworkInterfaceKey(plugin: "container-network-vmnet", variant: "reserved")] = NonisolatedInterfaceStrategy(log: log)
	}

I noticed the FIXME: comment. I interpret it to mean what I am requesting in this feature... that there be a system property or config that can be used to identify the supported network plugins and their associated interface strategy.

Could this be handled with new entries in config.toml?
@katiewasnothere

I figured this might be a good initial contribution.

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