feat(signals): add mergeUpdaters utility function

[MillerSvt]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/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?

Closes #4783

What is the new behavior?

Add ability to merge updaters

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[rainerhahnekamp]

I've given my answer in #4783 (comment)

Maybe we should first check, if this feature is required.

[markostanimirovic]

Thanks for working on this, @MillerSvt! However, before opening the PR, the feature request needs to be accepted first. More info is available in our contribution guidelines.

We'll consider your feature request and let you know if it will be accepted or not. Until then, I'm going to close this PR.