rgw/pubsub: avoid allocating hash set of strings for attr search

rgw/pubsub: avoid allocating hash set of strings for attr search

this unordered_set was not static, so we reinitialized it on every call

replace with a constexpr array of string_views so we can search through
sequential memory that's laid out at compile time

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