[FEATURE]: Predicates i18n for unique fields

[AoAnima]

Use case

When using i18n predicates, such as those in Cyrillic, for unique fields, these predicates are added without <> in the addQueryIfUnique function, leading to a query error.

I propose adding the following code to dgraph/graph/server.go at line 1744 before
if !strings.HasPrefix(pred.ObjectId, "val("){....}

such a code

if !isLatinString(predicateName) {
				predicateName = fmt.Sprintf(`<%v>`, predicateName)
}

func isLatinString(s string) bool {
	for _, r := range s {
		if !unicode.IsLetter(r) || !unicode.Is(unicode.Latin, r) {
			return false
		}
	}
	return true
}
func addQueryIfUnique(qc *queryContext) error {
...

                      if !isLatinString(predicateName) {
				predicateName = fmt.Sprintf(`<%v>`, predicateName)
			}
			if !strings.HasPrefix(pred.ObjectId, "val(") { 
                              ....
                        }

}

Links to Discuss, RFC or previous Issues and PRs

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Links to examples and research

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Current state

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Solution proposal

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Additional Information

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