Ability to derive a `Signal` while dropping some values

## Specific Demand

Analogous to `map` (#1537), I'd like a way to produce a new signal that contains only a subset of values set in the original signal, potentially mapped over to a new type, `U`. This analogous to Rust's `filter_map` semantics

cf. discussion at https://github.com/DioxusLabs/dioxus/pull/1537#issuecomment-1769205030

## Implement Suggestion

```rust
fn signal_filter_map(cx: Scope, sig: Signal, initial: U, f: F) -> Signal
where
F: Fn(T) -> Option + 'static,
T: Copy,
{
let res: Signal = use_signal(cx, || initial);
dioxus_signals::use_effect(cx, move || {
let value = *sig.read();
if let Some(value) = f(value) {
res.set(value);
}
});
res
}
```