[Feature] Add system of annotations to the parser

Thanks to @gdupont1, we thought about adding a system of annotations to the parser to give the possibility to extend our parsing to external developers.

**Question.** @jbcaillau Why do we have to write `==` instead of `=` for the constraints?

**Remark.** Another idea is to provide an intermediate structured data between the definition of the optimal control problem and the `OptimalControlModel` to facilitate manipulations such as adding a state, normalizing the time, etc.