[GeoMechanicsApplication] Add serialization to interface element

[rfaasse]

The current U interface element in GeoMechanicsApplication does not implement serialization. For use of the orchestrator for staged construction analysis it needs to be there. Preferably before the interface element is extended with water pressure D.o.F. (a suggestion is to do it in parallel with the Pw addition)

Serialization is needed for:
std::function<Matrix(const Geometry&, const array_1d<double, 3>&)> mfpCalculateRotationMatrix;
std::unique_ptr mpIntegrationScheme;
std::unique_ptr mpStressStatePolicy;
std::vectorConstitutiveLaw::Pointer mConstitutiveLaws;
IntegrationCoefficientsCalculator mIntegrationCoefficientsCalculator;

and when added the water pressure geometry member variables.
The mfpCalculateRotationMatrix can be restored from the knowledge GetGeometry().LocalSpaceDimension() i.s.o. serialized. The others can be serialized as shown in U_Pw_base_element.cpp

Serializing a std::optional mpOptionalPressureGeometry requires modifications in Kratos Core to accommodate std::optional