Fix Extension encoded length

[overhacked]

Extension is odd compared to other messages because its Unparsed payload does not include a length field. The implementation of Encode::encode for Extension accounts for this by writing the bytes directly from the Vec<u8> member of Unparsed, but the implementation of Encode::encoded_len for Extension did not account for the 4 extra bytes (a u32) that <Vec<u8> as Encode>::encode normally prepends.

Therefore, an Extension message on the wire claims it has 4 more bytes than its payload actually contains, which hangs most SSH agent server implementations waiting for the 4 additional bytes to arrive.

[wiktor-k]

Thanks for the fix! 🎉

Would you be able to fix the sign-off? (it should be as easy as git commit --amend --signoff && git push --force-with-lease)

Now it occurred to me that we didn't have a test vector for an extension in our test cases so I need to add one really fast 😅

[wiktor-k]

@overhacked I've filed a different PR solving the same bug: https://github.com/wiktor-k/ssh-agent-lib/pull/66/files

The difference is that I've adjusted the encoded_len of the Unparsed (which is where I think the fix should be since it's Unparsed that has the special logic) and adjusted Extension::encoded_len to use that implementation (it should be doing that from the beginning).

Additionally I've added an extension binary message for tests and fixed the tests to test for correct values of encoded_len.

I hope you don't mind me taking the liberty of fixing it in a different way and appending your name to the commit (with Co-authored-by). If there are any concerns I'm all 👂s

👋

[overhacked]

Closing in favor of #66.