Add VLAN support

[DerFetzer]

This PR adds basic VLAN support.

There are probably some things missing since I am not yet that familiar with the codebase. I hope that you can point me in the right direction.

If you add the VLAN config option to the loopback example it works and you can see the tags in the output as well as in the recorded pcap file.

[codecov]

Codecov Report

Attention: Patch coverage is 56.65914% with 192 lines in your changes are missing coverage. Please review.

Project coverage is 79.66%. Comparing base (4c27918) to head (5616b69).

Files	Patch %	Lines
src/iface/interface/mod.rs	35.38%	84 Missing ⚠️
src/wire/vlan.rs	75.13%	47 Missing ⚠️
src/wire/ethernet.rs	0.00%	30 Missing ⚠️
src/iface/interface/ipv4.rs	6.89%	27 Missing ⚠️
src/iface/interface/ethernet.rs	92.72%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #902      +/-   ##
==========================================
- Coverage   79.96%   79.66%   -0.30%     
==========================================
  Files          82       83       +1     
  Lines       28378    28783     +405     
==========================================
+ Hits        22693    22931     +238     
- Misses       5685     5852     +167     

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[DerFetzer]

Ok, so first thing is to figure out how to integrate VLAN into existing tests for coverage.

[thvdveld]

Since src/wire/vlan/mod.rs is not that big, could you move that file to src/wire/vlan.rs? Could you also add some documentation in that file where we can find information about the frame formats? I'm not so familiar with VLAN. Thank you!

[DerFetzer]

@thvdveld I implemented your remarks a while back but forgot to write a new comment here.
What do you think about the current state of this PR? Thanks!

[thvdveld]

I'm not really comfortable merging this as I'm not familiar with the VLAN/Ethernet implementation. Maybe @Dirbaio or @whitequark could take a look?

[whitequark]

This is a large and complex PR and I won't be reviewing it as-is; splitting it into multiple parts, starting with the wire bits, may help.