Add option to send select/all Gateway events to specified endpoint(s) at guild level

[rollie42]

Currently Gateway events can be monitored via an application establishing a persistent connection and listening for such events. But for servers that change rarely, this requires having said application always running, which is not desirable for individual developers. Also, any downtime (planned or not) of said application may mean missed events, which may be important to the server in question. It would be possible to have multiple parallel instances of this application, but doing so means each event could be processed multiple times, which could cause a different class of problems.

Instead, it would be useful to allow a guild admin to specify a list of endpoints to receive gateway events, and optionally specify which events said endpoint should receive. This could be done either exclusively through the server settings and/or as part of the endpoint itself (i.e., require the endpoint to implement a 'gatewayIntents' path, which is required to respond with which gateway events it is interested in, which updates as part of a heartbeat).

This pattern better supports updates to an application not causing missed events, helps prevent duplication of handlers, increases visibility and control of server owners of who can receive event update, and also simplifies the interface for applications to receive such events.