refact(action): Make ActionSet behave as a set instead of a list

This switches the implementation of an ActionSet from a slice to a map
to ensure the elements in an ActionSet are unique. This allows for other
packages to perform set operations such as a union or difference without
needing to account for duplicates.

internal/tests/api/hostsets/host_set_test.go

@@ -146,6 +146,9 @@ func TestList_Plugin(t *testing.T) {
 			comparableSetSlice(ul.Items),
 			cmpopts.IgnoreUnexported(hostsets.HostSet{}),
 			cmpopts.IgnoreFields(hostsets.HostSet{}, "Version", "UpdatedTime"),
+			cmpopts.SortSlices(func(a, b string) bool {
+				return a < b
+			}),
 		),
 	)
 
@@ -165,6 +168,9 @@ func TestList_Plugin(t *testing.T) {
 			cmpopts.SortSlices(func(x, y *hostsets.HostSet) bool {
 				return x.Id < y.Id
 			}),
+			cmpopts.SortSlices(func(a, b string) bool {
+				return a < b
+			}),
 		),
 	)