new flag to build just the projects afected by the cross pull request

The idea behind this flag would be to just checkout and build/execute the projects affected by the cross PR set.

- This functionality affects to fdb and pr flows.
- New flag could be --justOpenPullRequest or --justPR (or both), better name ideas are welcome.
- this change affects to main and typescript/main branches
- new input/flag should be added to action.yml

Start by comparing the main and typescript/main branches and trace the existing fdb and pr flows. Update action.yml with the chosen input or flags, then verify that a run checks out and builds or executes only projects affected by the cross-PR set.

Written by the indexing model from the issue text.