feat: Deduce generic custom types in entry_points macro

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 17.64706% with 14 lines in your changes missing coverage. Please review.

Project coverage is 85.15%. Comparing base (e8ad3f5) to head (3504ae3).

Files	Patch %	Lines
sylvia-derive/src/message.rs	27.27%	8 Missing :warning:
sylvia/tests/api.rs	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #403 +/- ## ========================================== + Coverage 84.83% 85.15% +0.32% ========================================== Files 41 41 Lines 2921 2911 -10 ========================================== + Hits 2478 2479 +1 + Misses 443 432 -11 ```

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