Add C API for SparseObservable

[mtreinish]

What should we add?

The recently added SparseObservable class was added as a Rust native object in #12671 . To facilitate different use cases for generating and interacting with this new structure from compiled languages including other Python libraries' compiled extensions we should add a lower level C API around the structure. This will likely involve splitting the current SparseObservable structure into a pure rust struct and a wrapping python interface struct. Then we can add a new C FFI wrapper for it, likely should be put in a new crate c_ext crate or something like that to define the C API to qiskit. This sparse observable interface will likely be a good way to PoC the packaging and structure for other C API's that have a longer on-ramp, mainly: #13276.

[jakelishman]

There's a decent amount of comment in #13391 about how this could/should be done to make the clean Rust/Python split, and the C FFI would follow on from that.