Add methods for exposing collection entries as Observable<read-only-collection>

[nadako]

See #5...

• [x] ObservableList.Observe
• [x] ObservableDictionary.Observe

I'm going to test this in practical project a bit before merging, because there are some questions to answer, like:

• do we want to expose the interal storage (so, not further auto-observable) or the collection itself as read-only (so auto-observable).
• do we want a separate IReadOnlyObservableList/Dictionary interface for better code clarity?
• (implementation detail, but still need to check) is it enough to set valid=true on getting the value?

[codecov-commenter]

Codecov Report

Base: 99.12% // Head: 99.14% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (a88a1db) compared to base (e30665d). Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #15 +/- ## ========================================== + Coverage 99.12% 99.14% +0.01% ========================================== Files 19 20 +1 Lines 917 934 +17 ========================================== + Hits 909 926 +17 Misses 8 8 ``` | [Impacted Files](https://codecov.io/gh/nadako/TinkStateSharp/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dan+Korostelev) | Coverage Δ | | |---|---|---| | [src/TinkState/Runtime/Internal/ObservableList.cs](https://codecov.io/gh/nadako/TinkStateSharp/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dan+Korostelev#diff-c3JjL1RpbmtTdGF0ZS9SdW50aW1lL0ludGVybmFsL09ic2VydmFibGVMaXN0LmNz) | `100.00% <100.00%> (ø)` | | | [...nternal/ObservableList\_ObservableOfReadOnlyList.cs](https://codecov.io/gh/nadako/TinkStateSharp/pull/15?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dan+Korostelev#diff-c3JjL1RpbmtTdGF0ZS9SdW50aW1lL0ludGVybmFsL09ic2VydmFibGVMaXN0X09ic2VydmFibGVPZlJlYWRPbmx5TGlzdC5jcw==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dan+Korostelev). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dan+Korostelev)

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[nadako]

Well, it works and it fulfills my intended use cases so far, so let's merge it and change stuff later if new ideas/examples arise.