Add new socket type for raw ethernet protocols

[ususdei]

Hi everyone!

I'm currently working on a rust library for a protocol which runs on plain ethernet using a custom ethertype (TECMP on 0x99FE).
It also uses an additional IP-based protocol for discovery and configuration.

I would also like to use this library on some STM32, so I tried to port it from native linux sockets to the smoltcp stack.

Thus I started to implement some basic support for raw ethernet sockets.
This is still a work in progress but the basic functionality already works on an STM32F7.
It is still missing support for Metadata and the send API could be more convenient
but I would like to get this out there and gather some early feedback as to whether you would even consider merging something like this.

So just some initial questions:

• Is this feature even in scope for the smoltcp project?
• Do you agree with my overall design?
• Do you already have any other initial feedback?

Thank you all for your feedback and your great contribution to the embedded rust ecosystem!

[codecov]

Codecov Report

Attention: Patch coverage is 63.08725% with 110 lines in your changes missing coverage. Please review.

Project coverage is 81.03%. Comparing base (f8bfa31) to head (ef11c4f).

Files with missing lines	Patch %	Lines
src/socket/eth.rs	65.72%	85 Missing ⚠️
src/iface/interface/mod.rs	42.50%	23 Missing ⚠️
src/socket/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1013      +/-   ##
==========================================
- Coverage   81.17%   81.03%   -0.15%     
==========================================
  Files          81       82       +1     
  Lines       28956    29235     +279     
==========================================
+ Hits        23505    23690     +185     
- Misses       5451     5545      +94     

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