WAKU2-RLN-CONTRACT: introduce billing periods for memberships

[s-tikhomirov]

The initial version of WAKU2-RLN-CONTRACT only allows RLN memberships of certain length (a contract-wide parameter set by the contract Owner).

A proposal by @alrevuelta is to introduce billing periods:

• a billing period is 30 days (good for UX as monthly payments are familiar to users);
• a user may register a membership for an arbitrary number of billing periods (up to a certain limit, such as 180 days);
• extensions would also be possible for multiple billing periods.

Discussion origins are here and here.

These changes are left for more careful considerations for the next iteration of the spec.

Things to consider:

• implementation complexity costs;
• game-theoretic consequences (for example, an incentive to register just for one billing period and then extend, while extensions are currently free).