Feature: HoldAllOperator

A similar operator already exists `fallback_when_silent` which is also a firehosing operator, emitting at every tick, but with a fallback value instead of the latest cached value, which is what the hold operator will do, with the added configuration that it can have a limit on how long to hold something for.
This holding length is also coming from the upstream signal.

```rs
HoldRequest {
in: In,
hold_for: Duration
}
```