Add setStringParameter and expose initial_guess parameter in proxqp

[traversaro]

Fix #14 .

The setStringParameter was added to the interface, and implemented in the proxqp plugin to expose the initial_guess parameter. As that parameter is an Enum in the proxqp settings, to expose it easily with the solver-agnostic interface we expose it as a string parameter.

As adding a new method in the interface breaks any plugin not maintained in this repo, it is a breaking change and so we bump the minor version to 0.1.0 (see https://github.com/ami-iit/qpsolvers-eigen?tab=readme-ov-file#versioning-policy).

[traversaro]

As a first super non-scientific test, on Windows there is a ~10x speedup in the MPCUpdateMatricesTest (i.e. ctest -VV -R MPCUpdateMatricesTest) by setting the initial_guess setting to WARM_START_WITH_PREVIOUS_RESULT instead of the default, see:

Before setting initial_guess to WARM_START_WITH_PREVIOUS_RESULT:

4: [     cout ] Testing solver osqp
4: [     cout ] Average time = 0.00024 seconds.
4: [     cout ] Testing solver proxqp
4: QpSolversEigen::ProxqpSolver::setStringParameter: unknown value << THIS_IS_NOT_A_VALID_INITIAL_GUESS_VALUE for parameter with name: initial_guess
4: QpSolversEigen::ProxqpSolver::setStringParameter: unknown setting name: this_is_not_a_valid_proqp_parameter_name
4: [     cout ] Average time = 0.00202 seconds.

After setting initial_guess to WARM_START_WITH_PREVIOUS_RESULT:

4: [     cout ] Testing solver osqp
4: [     cout ] Average time = 0.00022 seconds.
4: [     cout ] Testing solver proxqp
4: QpSolversEigen::ProxqpSolver::setStringParameter: unknown value << THIS_IS_NOT_A_VALID_INITIAL_GUESS_VALUE for parameter with name: initial_guess
4: QpSolversEigen::ProxqpSolver::setStringParameter: unknown setting name: this_is_not_a_valid_proqp_parameter_name
4: [     cout ] Average time = 0.00028 seconds.