fix(store): support using createActionGroup with props typed as unions

[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 current behavior?

Closes #3712

What is the new behavior?

We are able to use the createActionGroup function with props typed as unions:

const booksApiActions = createActionGroup({
  source: 'Books API',
  events: {
    'Load Books Success': props<{ books: string[]; status: 'success' } | { books: Book[] }>(),
  },
});

// works as intended
booksApiActions.loadBooksSuccess({ books: [''], status: 'success' });
booksApiActions.loadBooksSuccess({ books: [{ id: 1, title: '' }] });

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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