refactor: improve Hook Hints typing

[gruebel]

This PR

• it doesn't really enforce 4.2.2.1, but a type checker will complain, if you try to change a value in the hook hints
• had to switch from dataclass to a normal class HookContext to make the metadata fields immutable

[codecov[bot]]

Codecov Report

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

Project coverage is 94.71%. Comparing base (141858d) to head (ee982c1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #285 +/- ## ========================================== + Coverage 94.52% 94.71% +0.19% ========================================== Files 18 18 Lines 493 492 -1 ========================================== Hits 466 466 + Misses 27 26 -1 ``` | [Flag](https://app.codecov.io/gh/open-feature/python-sdk/pull/285/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/open-feature/python-sdk/pull/285/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | `94.71% <100.00%> (+0.19%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature#carryforward-flags-in-the-pull-request-comment) to find out more.

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