resip: SSL: move method for reading a file into rutil/Data

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10469 ddefafc4-47db-0310-ae44-fa13212b10f2
repro: allow specification of arbitrary domain cert/key PEM files

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10470 ddefafc4-47db-0310-ae44-fa13212b10f2