build: import config from imap-codec

[soywod]

This PR brings config from imap-codec: just, nix, github actions, rustup, rustfmt etc.

[duesee]

Hm... not sure what to think about this PR. At first I thought it was merely a copy of some files from imap-codec -- this would be fine! But then, there was also content that... well... appeared to be LLM-generated, such as the non-existent ext_ensions? Also, I think we should not make the impression this crate is in a good state (yet) since I am myself not even sure what works and what not -- long time no see :-) Still... the direction is right. Let's first bring in the CI, then continue to make progress!

For the future, let's try to be more precise in what is (almost) a copy only, what is a reviewed contribution from you, and what is generated (LLM) code. I trust you as a person and as an engineer, but when LLMs gets mixed in, well, I basically start to distrust every line to a point that even when you say "this is a copy", I would double-check.

[soywod]

I should have been more precise about the context, my apologies.

imap-codec and types are already used inside Pimalaya. I would love to have the exact same tools but for SMTP. Unfortunately we don't have time both of us to dig into it. So I initiated a naive "clone from Claude", where the instructions were: imap-codec rocks, please do the same for SMTP. The result is available at https://github.com/pimalaya/smtp-codec.

We cannot trust what is generated at the moment. It only seems to work (I was able to connect to SMTP servers and send few commands with success). So I thought to bring chunk of PRs so we can discuss on them, adjust and merge what is relevant.

Now, regarding this PR, most of the things are copies except the stupid generated documentation that I should not have committed. My bad, I just reverted them. Regarding the cargo features, they are part of the generated fake smtp-codec, it was also a mistake of mine, they should not have been committed in this PR.

Let's focus on this PR first, as it is the easiest part we could start on. Could you please enable CI so I can make it work?