Expose read only collections for source cache

[RolandPheasant]

Similar to the recent PR to expose list items as a read only list, this PR changes the observable cache to use readonly collections.

We now have theses exposed

IReadOnlyList<TObject> Items { get; }
IReadOnlyList<TKey> Keys { get; }
IReadOnlyDictionary<TKey, TObject> KeyValues { get; }

instead of raw IEnumerables

[github-actions]

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