feat(signals): add SignalStoreFeatureType

[rainerhahnekamp]

This allows custom features to reuse another feature's inferred state, props, and methods as input without duplicating the feature result type manually.

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 #5185

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No