Support objective offset

[klamike]

Currently we only support c'x, not c'x + c0. For MOI we handle this in the extension, but it might be better for that code to be in CoolPDLP proper. Relevant for #22

[gdalle]

That's a very easy fix, I'd like to try my hand at unleashing a coding agent on it if you don't get to it before me

[klamike]

You might be able to just tag @copilot here for it to make a PR, if you want to give it a shot. Though I prefer running it locally and making the commits myself.