Support different time periods for different repos for report-config.yaml

[lyuanww]

What feature(s) would you like to see in RepoSense

With the the one-stop config feature completed (PR: #2299) , we can enhance the feature by supporting different time periods for different repos for more customisability.

Optimally, the since-date and until-date fields should be optional.

We have already implemented this feature (refer to #2280) for the CSV dates.

Additional context

Will need to take note of resolving conflict for the dates between different configurations (CLI, CSV and report-config.yaml)

Please refer to #2334 for more context.