Added Adjoint Integrator

[BBhattacharyya1729]

Added new type of bilinear integrator that applies the exponential of a joint (direct sum) Hamiltonian on a stack of unitaries. This allows for simulating the dynamics of sensitivities.

[andgoldschmidt]

I think we can avoid needing this PR by pushing the burden to quantum integrators for now, see the PR in QC harmoniqs/QuantumCollocation.jl#190

function VariationalKetIntegrator(
    sys::VariationalQuantumSystem,
    traj::NamedTrajectory, 
    ψ̃::Symbol, 
    ψ̃_variations::AbstractVector{Symbol},
    a::Symbol
) 
    var_ψ̃ = hcat(ψ̃, ψ̃_variations...)
    G = a -> Isomorphisms.var_G(sys.G(a), [G(a) for G in sys.G_vars])
    return BilinearIntegrator(G, traj, var_ψ̃, a)
end

[andgoldschmidt]

Moved to #9 with updates