feat: Optionally add timestamp assertions for update manifests

[ok-nick]

Introduces a update_manifest_timestamp_assertion setting, which automatically generates a timestamp assertion for update manifests. This is particularly useful when timestamping manifests that are signed offline.

• Closes Timestamp Assertion #1210

[codspeed-hq]

CodSpeed Performance Report

Merging #1603 will not alter performance

_{Comparing ok-nick/timestamps (fa7b342) with main (629ab6e)}

Summary

✅ 16 untouched
⏩ 2 skipped¹

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩