Move topology inputs away from futures::Sink

[lukesteensen]

Currently the Fanout type that is core to Vector's data plane handles generic targets that adhere to the futures::Sink trait. This is a trait we've been slowly trying to remove from the codebase for some time due to its complexity, potential for implementation bugs, and less-than-ideal performance.

For those reasons, we'd like to replace the GenericEventSink type alias in Fanout with a simpler, concrete type that exposes an API better suited to our needs there. The main methods that'd be helpful would be:

1. fn try_send(Event) -> Result<(), Event>: try to send an event immediately, returning it if the downstream is not ready to accept it
2. async fn send(Event) -> Result<(), ClosedError>: send an event downstream, yielding the current task if it is not yet ready and only returning an error when the downstream has permanently closed

This would have a few advantages for Fanout specifically:

• The try_send method would make it simple to first send to any ready inputs (the goal of the work around chore: Remove the Sink impl from Fanout #11206 and friends) and only wait to send to sinks that actually aren't ready yet
• Error handling is simplified by only considering a single case that's always essentially fatal
• Conceptually simpler not to deal with poll_* methods