Add private runner minutes to Historical CI Minute charts

Problem

In #246844 (closed), we enabled shared runner minutes historical tracking for CI minute usage. Users want to be able to see private runner minutes as well

Proposal

We could apply the same proposal as per #254231 (closed) but for a different database column:

• add new column ci_namespace_monthly_usages.private_runners_minutes_duration
• add new column ci_project_monthly_usages.private_runners_minutes_duration
• change UpdateBuildMinutesService to accumulate the duration for jobs executed by private runners. E.g. CI minutes consumption would be 0 since the runner cost factor is 0 but the duration should be accumulated in minutes anyway in the respective table column.

If CI minutes monthly limit is set (Ci::Minutes::Quota#enabled?) we allow this duration to be tracked.