rgw/acl: ACLGrant uses variant for grantee types

rgw/acl: ACLGrant uses variant for grantee types

use of `ACLGrant::get_id()` was awkward because most grantee types
returned nothing, but emails were returned as `struct rgw_user`. change
the internal representation into a variant, and expose getters for each
grantee type so callers can handle each type specifically. the encoded
format of `ACLGrant` remains unchanged

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