Allow secret generators to access order datetime

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.08%. Comparing base (11a6390) to head (63be55c). Report is 10 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4110 +/- ## ========================================== - Coverage 78.08% 78.08% -0.01% ========================================== Files 429 429 Lines 60252 60254 +2 ========================================== + Hits 47047 47048 +1 - Misses 13205 13206 +1 ``` | [Files](https://app.codecov.io/gh/pretix/pretix/pull/4110?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix) | Coverage Δ | | |---|---|---| | [src/pretix/base/secrets.py](https://app.codecov.io/gh/pretix/pretix/pull/4110?src=pr&el=tree&filepath=src%2Fpretix%2Fbase%2Fsecrets.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pretix#diff-c3JjL3ByZXRpeC9iYXNlL3NlY3JldHMucHk=) | `94.21% <50.00%> (-0.75%)` | :arrow_down: |