Add new param to forbid adding new members

[yngvar-antonsson]

This pull request introduces a new feature to manage the addition of new members in the membership system. The most important changes include adding a global parameter to control the addition of new members, updating the init function to accept this parameter, and modifying the members.set function to respect this setting.

New Feature: Control Addition of New Members

• membership.lua: Introduced _global_params to store global configuration parameters and updated the init function to accept forbid_new_members parameter. This parameter controls whether new members can be added to the group. [1] [2]
• membership.lua: Updated the _protocol_step function to respect the forbid_new_members setting when updating member information. [1] [2]

Codebase Enhancements

• membership/events.lua: Added global_params to manage global configuration and updated the events.handle function to respect the forbid_new_members setting. [1] [2]
• membership/members.lua: Enhanced the members.set function to skip adding new members if forbid_new_members is set to true and added logging for skipped members. [1] [2]
• membership/stash.lua: Initialized global_params with a default value for forbid_new_members.