Add a pure C interface

[jgillis]

Hi! I'd be interested in interfacing elastiqp with CasADi.
Since we care about C codegeneration, such interface works best when the solver has a C API.

Here is one modeled on daqp's C interface.

[danielpmorton]

Hi @jgillis, great to get in touch! I'm so impressed by your work on CasADi and it would be quite cool to have an CasADi/ElastiQP interface. I ran a few tests on your PR locally and things are looking good already. I'll take a deeper look through the code and get back to you shortly!

[jgillis]

I was working on an experimental slack extension for our nlpsol and conic interfaces, so your elastiQP solver would be a great first fit.