feat(signals): add entityConfig function

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

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

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

const Store = signalStore(
  withEntities({ entity: type<Todo>(), collection: 'todo' }),
  withEntities({ entity: type<User>(), collection: 'user' }),
  withMethods((store) => ({
    addTodo(todo: Todo): void {
      patchState(store, addEntity(todo, { collection: 'todo', selectId }));
    },
    addUser(user: User): void {
      patchState(store, addEntity(user, { collection: 'user' }));
    },
  }))
);

Closes #4393

What is the new behavior?

const todoConfig = entityConfig({
  entity: type<Todo>(),
  collection: 'todo',
  selectId: (todo) => todo.key,
});

const userConfig = entityConfig({
  entity: type<User>(),
  collection: 'user',
});

const Store = signalStore(
  withEntities(todoConfig),
  withEntities(userConfig),
  withMethods((store) => ({
    addTodo(todo: Todo): void {
      patchState(store, addEntity(todo, todoConfig));
    },
    addUser(user: User): void {
      patchState(store, addEntity(user, userConfig));
    },
  }))
);

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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