rgw/auth: perm_mask uses std::optional

rgw/auth: perm_mask uses std::optional

resolves a compiler warning about an uninitialized variable in
boost::optional:

In file included from ../src/rgw/rgw_common.h:1508,
                 from ../src/rgw/rgw_main.cc:18:
../src/rgw/rgw_auth.h: In member function ‘virtual rgw::auth::swift::DefaultStrategy::aplptr_t rgw::auth::swift::DefaultStrategy::_ZThn56_NK3rgw4auth5swift15D
efaultStrategy15create_apl_turlEPN4ceph6common11CephContextEPK9req_stateRK11RGWUserInfo(ceph::common::CephContext*, const req_state*, const RGWUserInfo&) cons
t’:
../src/rgw/rgw_auth.h:652:38: warning: ‘<unnamed>.boost::optional<unsigned int>::<unnamed>.boost::optional_detail::tc_optional_base<unsigned int>::m_storage’
may be used uninitialized [-Wmaybe-uninitialized]
  652 |       this->perm_mask = perm_mask.get();
      |                         ~~~~~~~~~~~~~^~
In file included from ../src/rgw/rgw_rest_swift.h:14,
                 from ../src/rgw/rgw_main.cc:24:
../src/rgw/rgw_swift_auth.h:27:73: note: ‘<anonymous>’ declared here
   27 |     : LocalApplier(cct, user_info, LocalApplier::NO_SUBUSER, boost::none) {
      |

Signed-off-by: Casey Bodley <cbodley@redhat.com>