PoC on DORA metric configuration object

Summary

Based on #358385

We need to provide PoC for DORA metrics configuration which allows us to alter metrics calculation behaviour. This PoC should be used for Dogfooding on gitlab.com itself.

Proposal

Create a feature flag which enables alternative behaviour of DORA metrics calculation for particular project. With it enabled only MRs targeting main or master branch while calculating Lead Time For Changes metric. To reduce scope of this PoC several assumptions and simplifications has been made:

1. No data migration is supported. All old data will stay as it was before enabling feature flag and only new gathered data will be calculated with new approach. Same applies to disabling feature flag.
2. No UI for configuration is required. Hardcoded configuration should mark main and master as target branch to filter out project MRs.
3. Only project level should be supported for now. No propagation to group level is required.

/cc @blabuschagne