Add {{mcs_boresight}} join column in {{mcs_exposure}}

We have a (currently unused) mcs_boresight table, which should get a new row whenever that boresight changes. For that to be useable, the mcs_exposure table needs a join column to mcs_boresight.

The primary key for mcs_boresight is pfs_visit_id. I guess we can use that. Or since all of this is unused we could restructure.