feat(signals): provide ability to use interface as state type

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #4117

What is the new behavior?

We can use interfaces as state/nested slice types:

interface User {
  id: number;
  name: string;
}

interface State {
  user: User;
}

const state = signalState<State>({
  user: { id: 1, name: 'Marko' },
});

state.user // type: DeepSignal<User>
state.user.name // type: Signal<string>

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[timdeschryver]

Before approving/merging, do we want to update the records to objects first?