Improve utils of slices (#22379)

- Move the file `compare.go` and `slice.go` to `slice.go`.
- Fix `ExistsInSlice`, it's buggy
  - It uses `sort.Search`, so it assumes that the input slice is sorted.
- It passes `func(i int) bool { return slice[i] == target })` to
`sort.Search`, that's incorrect, check the doc of `sort.Search`.
- Conbine `IsInt64InSlice(int64, []int64)` and `ExistsInSlice(string,
[]string)` to `SliceContains[T]([]T, T)`.
- Conbine `IsSliceInt64Eq([]int64, []int64)` and `IsEqualSlice([]string,
[]string)` to `SliceSortedEqual[T]([]T, T)`.
- Add `SliceEqual[T]([]T, T)` as a distinction from
`SliceSortedEqual[T]([]T, T)`.
- Redesign `RemoveIDFromList([]int64, int64) ([]int64, bool)` to
`SliceRemoveAll[T]([]T, T) []T`.
- Add `SliceContainsFunc[T]([]T, func(T) bool)` and
`SliceRemoveAllFunc[T]([]T, func(T) bool)` for general use.
- Add comments to explain why not `golang.org/x/exp/slices`.
- Add unit tests.

cmd/admin.go

@@ -950,7 +950,7 @@ func parseSMTPConfig(c *cli.Context, conf *smtp.Source) error {
 	if c.IsSet("auth-type") {
 		conf.Auth = c.String("auth-type")
 		validAuthTypes := []string{"PLAIN", "LOGIN", "CRAM-MD5"}
-		if !contains(validAuthTypes, strings.ToUpper(c.String("auth-type"))) {
+		if !util.SliceContainsString(validAuthTypes, strings.ToUpper(c.String("auth-type"))) {
 			return errors.New("Auth must be one of PLAIN/LOGIN/CRAM-MD5")
 		}
 		conf.Auth = c.String("auth-type")