Restore clock iteration conditional to Reactant extension timestepping

[dkytezab]

Currently, incorporating 'paranoid' state updating breaks inside of Reactant compilation, so the following code snippet, present in RK3 and QB2, is deleted from their Reactant-friendly counterparts.

if model.clock.iteration == 0
    update_state!(model, callbacks; compute_tendencies=true)
end

This has the disadvantage of requiring two updates to the body of either timestepper, rather than one. Ideally, we should extend traced_type_inner inside of OceananigansReactantExt to handle RectilinearGrid and other Oceananigans types, which is a prerequisite to using the @trace macro in the snippet above.

[glwagner]

An intermediate solution if nested tracing proves too challenging is to define a small interface here:

function first_step_update_state!(model)
    if model.clock.iteration == 0
        update_state!(model)
    end
    return nothing
end

# in extension
first_step_update_state!(model::ReactantModel) = nothing