feat(signals): allow returning an array of observables from withEffects

[markostanimirovic]

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• Tests for the changes have been added (for bug fixes / features)
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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:

Closes #5000

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[rainerhahnekamp]

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