chore: 🏷️ Update cacheModifier types

[pcbowers]

Technically, cacheModifier doesn't take an Observable since you're working with the actual value. This updates that type to make cacheModifier more usable/type-friendly.

[angelnikolov]

Hey, you are right, thanks!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (64c2728) 76.99% compared to head (0b3ecf4) 76.99%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #139 +/- ## ======================================= Coverage 76.99% 76.99% ======================================= Files 13 13 Lines 326 326 Branches 83 83 ======================================= Hits 251 251 Misses 60 60 Partials 15 15 ```

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