Option to specify optimization backend

[bagibence]

While NeMoS is moving away from JAXopt as its optimization backend, the solver interface is compatible with both JAXopt and Optimistix.

At first JAXopt will be kept as the default, but there could be a way for users to specify which optimization backend they want to use. (Currently this can be done in #365 by overwriting the solver registry, but it's hidden from users.)

There are multiple ways this could be done:

• By setting an environment variable such as NEMOS_SOLVER_BACKEND=optimistix like it's currently done in the tests.
• Giving the option to specify the backend for each algorithm.
  Notation for this could GLM(solver_name="LBFGS[jaxopt]") or GLM(solver_name="JaxoptLBFGS")