netdog: refactor to prepare for upcoming additions

[zmrow]

Issue number: Related to #2204

Description of changes: This refactor is in preparation for the upcoming additions related to static addressing.

Almost no code was changed minus the few clippy lints that are in the last commit. The majority of the code was moved around as is.

    netdog: Extract lease parsing functions to module

    netdog: refactor cli code into modules
    
    Since `main.rs` was starting to get large and unwieldy, and additional
    code is expected to be added, this commit refactors all the CLI code
    into separate files grouped under a `cli` module.

    netdog: implement the suggested clippy lints

Testing done:

• Unit tests continue to pass
• Build and run an aws-k8s-1.21 image successfully (current_ip, resolv.conf, primary_interface, and /etc/sysctl.d/90-primary_interface.conf all get properly written)

bash-5.1# cat /etc/sysctl.d/90-primary_interface.conf 
-net.ipv6.conf.eth0.accept_ra = 2
-net.ipv4.conf.eth0.rp_filter = 2

• Build and run a metal-dev image successfully (net.toml correctly parsed and all above files written)
• Manually run netdog's settings generator commands and observe proper output

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.