feat(signals): replace idKey with selectId when defining custom entity ID

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/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?

The custom entity id is defined using the idKey property:

const TodosStore = signalStore(
  withEntities<Todo>(),
  withMethods((store) => ({
    addTodo(todo: Todo): void {
      patchState(store, addEntity(todo, { idKey: '_id' }));
    },
  })
);

Closes #4217, #4392

What is the new behavior?

The custom entity id is defined using the selectId function:

const selectId: SelectEntityId<Todo> = (todo) => todo._id;

const TodosStore = signalStore(
  withEntities<Todo>(),
  withMethods((store) => ({
    addTodo(todo: Todo): void {
      patchState(store, addEntity(todo, { selectId }));
    },
  })
);

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This is not considered as a breaking change because the @ngrx/signals package is in developer preview.

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