Interception of published events for processing

[Semisol]

Is your feature request related to a problem? Please describe.
I think that interception of all incoming events would be useful for:

• people creating their own filters for spam
• dynamic whitelists
• pay-per-event relays

Describe the solution you'd like
There should be a way for any app to connect to the relay (gRPC, WS, doesn't matter, and could probably have authentication) and hook into all events being sent with the following information included:

• IP address (should account for X-Forwarded-For)
• The event itself
• the origin header

and the connected client could answer with one of the following:

• accept
• silently drop (OK as if nothing happened but do not persist and relay)
• drop

in the future, AUTH could be required. there should be a configuration option to block all event publishing if there is no connected client that is intercepting the events. (such as for a dynamic whitelist, I wouldn't want to allow non-whitelisted people from posting because the script I wrote went down)

Describe alternatives you've considered
None

Additional context
In the future, this could be extended to more things.