pkg/endpoint: create slice with capacity
pkg/endpoint: create slice with capacity

Be more efficient in creation of slice of identities in GetPolicyModel, as we
know how many entries will be added to it.

Signed-off by: Ian Vernon <ian@cilium.io>

tgraf/cilium,cilium/cilium,michi-covalent/cilium,eloycoto/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,scanf/cilium,tklauser/cilium,tgraf/cilium,scanf/cilium,tklauser/cilium,eloycoto/cilium,scanf/cilium,cilium/cilium,michi-covalent/cilium,eloycoto/cilium,eloycoto/cilium,cilium-team/cilium,tgraf/cilium,scanf/cilium,eloycoto/cilium,cilium/cilium,scanf/cilium,eloycoto/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium