[Feature] Adjust natural time boundaries for forget

[ccmgr]

Output of restic version

restic 0.16.4 compiled with go1.21.6 on windows/amd64

What should restic do differently? Which functionality do you think we should add?

Ability to override the default forget time boundaries
For example Monthly to be First day or Last Day (current default)

End of Week to be Friday or Saturday

What are you trying to do? What problem would this solve?

Backups that run out of hours between 00:00 and 08:00 will be used as retained backup for the month/year
Data changes on the last day for the month/year would be in the first snapshot of the following month/year
Thus Forget Month/Year would not have the last days data when backup completes after 00:00
Add an option to override Default Boundaries

--forget-monthly-boundary First/Last

Did restic help you today? Did it make you happy in any way?

Only started to migrate from Duplicati for backups with large quantity of small files

Much quicker to scan daily/hourly changes the SQLite DB of Duplicati

[eharris]

I would love to see this too, as I run my longer-term backups at the beginning of the month/week/day and would like to have the selection keep the oldest rather than the most recent within the given period.