feat(signals): add upsert entity updaters

[markostanimirovic]

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:

upsertEntity

Adds or updates an entity in the collection.
When updating, it does not replace the existing entity but merges it with the provided one.

patchState(store, upsertEntity(todo));

upsertEntities

Adds or updates multiple entities in the collection.
When updating, it does not replace existing entities but merges them with the provided ones.

patchState(store, upsertEntities([todo1, todo2]));

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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