WAKU2-RLN-CONTRACT: allow membership extension from Active state

[s-tikhomirov]

The initial version of WAKU2-RLN-CONTRACT only allows membership extensions from the GracePeriod state. Extensions from Active state are forbidden. However, from the product perspective, a service shouldn't stop the user if they want to commit to using the service for longer (even before its current term is about to expire).

For the future version of the spec, consider allowing membership extension from Active state.

Analyze the potential increased code complexity and game-theoretic consequences.

For the reference: the discussion started here (cc @richard-ramos and @lrevuelta).