Use single LinearConstraintProjection in ProximalProjection

[YigitElma]

For now, just a discussion for the appropriate API.

• Maybe add a _eq_solve_objective property to ProximalProjection which will be LinearConstraintProjection of ForceBalance and fixed boundary constraints.
• Only update the fixed parameter constraint targets of _eq_solve_objective, and calculate particular solution without factorization
• Pass _eq_solve_objective to eq.perturb and eq.solve?

Resolves #1404

[dpanici]

Add kwarg eq.solve which takes in LinearConstraintProjection called linear_constraint, which gets passed in through options of optimizer.optimize and from there it will skip necessary builds etc