refactor(transparent-proxy): allow config to be initialized

[bartsmykla]

Summary

Introduced InitializedConfig structure

InitializedConfig extends the Config struct by adding fields that require additional logic to retrieve their values. These values typically involve interacting with the system or external resources.

An InitializedConfig can contain nested configuration structs. If these nested structs share field names with the Config struct, they should embed the corresponding field from Config during initialization. I.e.

type Redirect struct {
	// NamePrefix is a prefix which will be used go generate chains name
	NamePrefix string
	Inbound    TrafficFlow
	Outbound   TrafficFlow
	DNS        DNS
	VNet       VNet
}

type InitializedRedirect struct {
	Redirect
	DNS InitializedDNS
}

func (c Redirect) Initialize() (InitializedRedirect, error) {
	var err error

	initialized := InitializedRedirect{Redirect: c}

	// .DNS
	initialized.DNS, err = c.DNS.Initialize()
	...

	return initialized, nil
}

I moved obtaining DNS servers from /etc/resolv.conf and getting loopback network interface name to initialization methods for appropriate sub-configs.

Combined static variables in builder_restore.go

By combining them all together at top it makes it easier to read.

Moved iptables/ip6tables consts to consts package

As we have a package for consts let's use it everywhere

Got rid of unnecessary IPTables struct

This structure was unnecessary as it was only used in BuildIPTablesForRestore, which also was using only its one method BuildForRestore. I moved logic of this method directly inside BuildIPTablesForRestore, which removes unnecessary complexity.

Checklist prior to review

• Link to relevant issue as well as docs and UI issues
  • Add a more systematic way to add transparent-proxy config #8324
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS
  • It won't
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • Updated unit tests + tested on local docker container
  • Don't forget ci/ labels to run additional/fewer tests
• Do you need to update UPGRADE.md?
  • There is no need
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label)
  • There is no need