Add support for optimizer SBPLX from NLopt

[cpossel]

What should we add?

The optimizer SBPLX is an improved version of Nelder-Mead. It is implemented in the NLopt library. Since the current code here in qiskit-machine-learning only includes the global optimizers from NLopt, i'd like to include also this specific optimizer.
Whether to include further optimizers while doing these changes might be an option to consider.

I already added all the code to the not actively maintained repo qiskit-algorithms (see qiskit-community/qiskit-algorithms#228; thanks to @woodsp-ibm for his help!), so it should be easy to adjust these changes here, too.