common/entity_name: print type names in get_valid_types_as_str()

0753b1d5326 ("common/entity_name: dump type name as string") turned
STR_TO_ENTITY_TYPE into a vector of (entity_type_t, std::string) pairs
but left get_valid_types_as_str() streaming .first, i.e. the uint8_t
type value, so e.g. `ceph-conf --name total.garbage` now says

  valid types are:  , \x01, \x04, \x02, \x10, \x08

instead of "auth, mon, osd, mds, mgr, client", and the
src/test/cli/ceph-conf/invalid-args.t cram test fails in run-cli-tests.
Stream the name (.second).

Fixes: 0753b1d53262212fbc8e8ebc32cf6b695bf1ddf4
Signed-off-by: David Galloway <david.galloway@ibm.com>
Assisted-by: Claude Fable 5 (claude-fable-5)