fix(store): support using factory selectors as extra selectors

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/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?

[x] Bugfix
[ ] 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 new behavior?

We can return factory selectors as createFeature.extraSelectors result:

export const usersFeature = createFeature({
  name: 'users',
  reducers: createReducer(initialState),
  extraSelectors: ({ selectUsers }) => ({
    selectUserById: (id: number) => createSelector(
      selectUsers,
      (users) => users.find((user) => user.id === id)
    ),
  }),
});

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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