feat(entity): strengthen typing of getInitialState

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• [x] 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 #4422

When initializing an EntityAdapter<T> and subsequently setting the initialState using ...adapter.getInitialState({ }), it is possible, given the current interface, to introduce unknown variables that do not conform to the user's specified state. This could result in bugs related to mistyped variable names, resulting in confusing user error scenarios.

What is the new behavior?

This small change modifies the interface to require that the input state S extends EntityState<T>, requires that the additionalState state argument conforms to an Omit version of this S type, which removes the 'id' and 'entities' properties by way of keyof EntityState<T>, and then finally, since the argument now already extends EntityState<T>, we can now simply return S.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

This reopens #4423 (with an update of the branch) - with the upcoming release this is a good timing imho.

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[GavynHolt]

Thanks very much for closing this out @timdeschryver!

[timdeschryver]

Thank you for bringing this up and opening a PR @GavynHolt!