feat(signals): Added selectable entities behaviour by default

[DJREMiX6]

• Updated withEntities to add selectedId and selectedEntity (even for named collections);
• Created selectEntity helper to select an entity given its ID.
• Created clearSelectedEntity helper to remove the currently selected entity without caring about its ID.
• Added Unit Tests
• Added documentation

Issue reference: #4717

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?

The withEntities() store feature does not allow the user to have a built-in system to select an entity.

Closes #4717

What is the new behavior?

Using withSignals() will expose a selectedId() signal and a selectedEntity() computed signal.
Two helpers can be used to update the selected entity:

• selectEntity()
• clearSelectedEntity()

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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