Vendor SerialCAN for offline builds

[dwarakeshbaraneetharan]

Replaces FetchContent (which cloned SerialCAN from GitHub on every clean build) with a vendored local copy. Build no longer requires internet access

[mdurrani808]

Is it possible to submodule this? Do we need the entire thing in source?

[dwarakeshbaraneetharan]

Is it possible to submodule this? Do we need the entire thing in source?

We could submodule it, but vendoring is more reliable as submodules break without --recurse-submodules, and if we ever need to re-clone on the Jetson in the field we'd need internet. Vendoring pins us to a known-working commit and it's only 2.9MB and we already vendor bigger things like the ZED wrapper and CTRE Phoenix headers. We don't need the full source tree though I can trim it down drop the examples/tests/docs if you want.

[IshanDutta11]

Excellent work, did a quick benchmark with my virtual machine's connection turned off for the original repo and your branch and now it seems to build without needing a connection.