Werk #13730: check_temperature: Make 'unique_name' and 'value_store' optional, but both are needed for trend computation

Since Checkmk version 2.1.0 the check_temperature function required {{unique_name}} and {{value_store}}. Both are only needed for trend computation.

Now {{unique_name}} and {{value_store}} are optional again. The check engine raises a ValueError if only one of both are set.

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