fix(signals): expose WritableStateSource to withFeature callback

[GuillaumeNury]

Fix #4766

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?

patchState cannot be called within withFeature.

Closes #4766

What is the new behavior?

patchState can be called within withFeature.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

I hesitated between feat and fix. Do not hesitate to ask me to change anything

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[rainerhahnekamp]

Good work. I left two comments for tiny changes.

[rainerhahnekamp]

Good job @GuillaumeNury! 👍

[markostanimirovic]

Thanks @GuillaumeNury!

[GuillaumeNury]

@rainerhahnekamp @markostanimirovic you're welcome! Thank you for the rebase and the fix. Sorry I was off for some days 🙈

[rainerhahnekamp]

All good @GuillaumeNury. We managed to do the last steps on our own 👍