[0.7] Implement Into<Signal<T>> & Into<MaybeSignal<T>> for stores and their fields

[brofrain]

Is your feature request related to a problem? Please describe.
MaybeSignal is a convenient wrapper for both dynamic and static values. All the primitive signals implement Into<MaybeSignal<T>> except for the newly added stores.

Describe the solution you'd like
All of the store-related types (Field, AtIndex, Subfield and Store) should be convertible to MaybeSignal using Into trait, so for example they can be used with #[prop(into)] value: MaybeSignal<T> syntax.

[brofrain]

@gbj
From my observations, converting a Memo / RwSignal / ... to Signal / MaybeSignal usually comes down to creating an inner SignalTypes wrapping the original signal. With reactive_stores a new approach may be needed, because reactive_graph is a dependency of reactive_stores, so SignalTypes cannot be expanded by a new variant.
Could you give me some hints on how to tackle this properly?

[gbj]

It can just use Signal::derive() and MaybeSignal::derive().

Something like this:

impl<T> From<Field<T>> for Signal<T> where T: Clone {
  fn from(value: Field<T>) -> Signal<T> {
    Signal::derive(move || value.get())
  } 
}