netdog: Support versioning in net.toml

[zmrow]

Issue number:
Implements #2214

Description of changes:

This change adds the ability to handle additional versions of `net.toml`
as the project continues to add features to network configuration.  A
top level `NetConfig` type has been removed in favor of a few traits.
Each new version of config is expected to implement these traits, the
most important of which converts the network configuration into
`WickedInterface` structs which are suitable for serializing directly to
file.

As part of this change, the `net_config` module has been lightly
refactored and split into submodules to make the pieces easier to use
among new versions.

Testing done:

• All unit tests continue to pass
• Build and boot an aws-k8s-1.21 variant and ensure the network config is correct
• Build and boot a metal-dev variant and ensure the network config is correct via a given net.toml

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

Since we now deserialize the majority of the network config to a toml::Value first (to handle the versions), this is stored as a String inside of the toml::Value, hence the change from &str to String.

[zmrow]

Nothing has really changed about the below code. This code was moved here into v1.rs since we use the NetConfigV1 struct for the simple configuration we get from the kernel command line.

[zmrow]

The above force push:

• changes the return type of deserialize_config from impl Interfaces to Box<dyn Interfaces> because we'll be returning different Interface implementors (netconfigv1, v2, etc). Rust considers them different types which means we need to use a Box rather than impl trait.
• Adds a small validate_config() function to simplify the match statement when deserializing config
• Implements the Validate and Interfaces traits for Box<T>