Implement activation events to reduce memory usage in the e-prop mechanism

[akorgor]

In the NEST implementation of the e-prop plasticity rule, all variables required to calculate weight updates are archived at every time step. When a spike activates an e-prop synapse, the archived history is retrieved from the postsynaptic neuron, and the used section is marked as no longer needed by that synapse in update_history. Once no synapse requires a specific part of the postsynaptic history, that part is deleted and the memory is freed. In biologically plausible scenarios, neurons often fire infrequently or stop firing during the learning process, causing their histories to accumulate indefinitely because they are never released. This PR resolves the issue by introducing activation events that, similar to spike events, trigger synapse activation so that obsolete history segments can be deleted, while not being sent to the postsynaptic neuron unlike spike events. Users can configure the interval for sending these activation events since the last spike.

• Introduce function pointers to improve efficiency.
• Test if the mechanism also works with static synapses.