rgw: convert string constant to string_view

rgw: convert string constant to string_view

instead of converting string constant to char*, construct string_views
from string constants

to silence GCC warnings like:

src/rgw/services/svc_sys_obj_cache.cc:512:7: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
  512 |     { "cache list name=filter,type=CephString,req=false",
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>