Implement advanced-step real-time iteration: AS-RTI

[FreyJo]

This pull request implements the advanced-step real-time iteration (AS-RTI)

It is implemented as a feature within the RTI module.
In order to use it, set the new options:

• as_rti_level
• as_rti_iter

The feedback phase of RTI and AS-RTI is the same.
The AS-RTI preparation phase is used instead of the "standard" RTI preparation phase, when the options above are set.

Additionally, this adds the flag:

• rti_log_residuals: which allows to log the residuals within (AS-)RTI. This was needed for the experiment in the paper, which reports the residuals of the inner iterations. Limited to convex-over-nonlinear and external cost modules for now.

The work in this PR is presented in the publication
"Advanced-Step Real-time Iterations with Four Levels -- New Error Bounds and Fast Implementation in acados" by Jonathan Frey, Armin Nurkanovic, Moritz Diehl
https://arxiv.org/abs/2403.07101

Limitations:

• AS-RTI-B not implemented for ns > 0.
• AS-RTI-C only implemented for linear inequality constraints.