@ngrx/component: Add ability to pass "non-Observable" values

[markostanimirovic]

It can be useful to have aliases in the template for nested properties. For example, the LetDirective could be used as follows:

<span *ngrxLet="form.controls.name.valid as isValid">{{ isValid }}</span>

The input type will be ObservableInput<T> | T which will also solve the typing issues mentioned in #3344.

Additional change to consider: Treat arrays as "non-Observable" values.

Current behavior:

@Component({
  selector: 'ngrx-array',
  template: `
    <!-- the value of `a` is equal to the last element of array -->
    <p *ngrxLet="array as a">{{ a }}</p>
  `,
})
export class ArrayComponent {
  array = [1, 2, 3, 4, 5];
}

Behavior with proposed change:

@Component({
  selector: 'ngrx-array',
  template: `
    <!-- the value of `a` is equal to array -->
    <p *ngrxLet="array as a">{{ a }}</p>
  `,
})
export class ArrayComponent {
  array = [1, 2, 3, 4, 5];
}

[brandonroberts]

Closed via #3440

[DibyodyutiMondal]

 <ng-container *ngrxLet="[timeSlice.mergeCells.month(period)] as months;">
   <div *ngFor="let monthCell of months; let i = index" [class.month-end]="i !== months.length - 1">
     {{monthCell.data.dt.monthShort}}, {{monthCell.data.dt.year}}
   </div>
</ng-container>

The function timeSlice.mergeCells.month(period) returns an array of objects that I want to iterate over.
But I also need the full array, because I need the array length to get the array's last item's index and use that to apply a certain class if it is the last item.

However, if I do not wrap the call in square braces and make an artificial array (like I have done above), then I cannot use ngFor inside the ngrxLet directive, because ngrxLet unwraps the array for me.

I assume this is intended behaviour, and the workaround is very very simple, but it would be nice to do this via an option to the directive, so that it becomes obvious to others what I'm trying to do.