Enable github code ownership for Jules

Prior to git migration, we had a [code owners file on MOSRS](https://code.metoffice.gov.uk/trac/jules/browser/main/trunk/CodeOwners.txt). We would like to enable a similar feature on github. This will result in automatic review requests being sent out whenever specific sub-directories are included in a PR.

The intention is to implement the change in two parts. The first will add owners - SSD plus other interested parties - to the technical sections of the repository. The second phase will add science section owners on an opt-in basis, using information in the MOSRS `CodeOwners.txt`.