Combined decorator for policy information

[lars-reimann]

What problem do you want to solve?

Combine the decorators add_rounding_spec and dates_active into a single decorator policy_info.

Todo

• [X] Pick an appropriate title.
• [X] Put Closes #XXXX in the first PR comment to auto-close the relevant issue once the PR is accepted. This is not applicable if there is no corresponding issue.
• [x] Document PR in CHANGES.md.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.57983% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.46%. Comparing base (df0b4b4) to head (d05c2bc).

Files	Patch %	Lines
src/_gettsim/shared.py	90.90%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #720 +/- ## ========================================== - Coverage 89.49% 89.46% -0.04% ========================================== Files 51 51 Lines 3637 3615 -22 ========================================== - Hits 3255 3234 -21 + Misses 382 381 -1 ```

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[lars-reimann]

I've deliberately kept test_dates_active unchanged, since it's more about the mechanism than the name of the decorator.