feat: barebones initial server-side upload-pack implementation

[JamesPatrickGill]

tl;dr

I am building a git forge and needed server-side git functionality. I found gitoxide, really liked it and decided to build on it. This is me attempting to upstream it.

I'm happy to rework it and take any feedback!

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Hey!

Sorry if me opening this PR out of the blue and is not the process but I have recently started building a small toy git forge in Rust for some fun and when it came to the "git" part I came across gitoxide and really liked it. However, I noticed that the server-side support was not ready for me to build what I wanted, this is my assumption from reading the code anyway.

So I was interested enough to just try to implement it myself and this is what this PR contains. I attempted to go as shallow as possible as to have something to maybe discuss if this is something gitoxide is interested in before I got carried away into all the nitty gritty. That being said it does do some (very little) real stuff:

What is here:

• gix-transport: Added server-side connection handling: connect message parser and Connection struct
• gix-protocol: Added server-side upload-pack: ref advertisement writers, wants/haves parser,
  ACK/NAK writers, and upload-pack orchestrators for both protocol versions
• gix-serve: New crate wiring it together: ref collection from a ref store, pack generation from wanted
  objects, and a top-level serve_upload_pack() entrypoint

There are a lot of caveats though...

Caveats:

• Ignoring "haves" so servers always send all objects reachable from wants
• Ignored capabilities for now in the actual serve implementation - they do get parsed though
• No add-on features supported i.e. shallow-clone etc
• The pack generation is as naive as it gets, just an unthreaded implementation

However despite being pretty far from done I have an ssh server in my git forge able to serve git clone requests - this can be seen here: https://github.com/JamesPatrickGill/steid

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I am very open to guidance and getting this ship shape if you see value. I attempted to match conventions from other crates in the repo as closely as possible.