Support `DUPLICATE_POLICY` on `ts.add`

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TS.ADD supports setting DUPLICATE_POLICY as well as/independently from ON_DUPLICATE. The two have separate behaviors -- DUPLICATE_POLICY sets the default for the key of TS.ADD does an implicit create, and ON_DUPLICATE overrides the existing default for the key on a particular add. It would be nice to support both.

Relevant issue from RedisTimeSeries to add documentation for this support: https://github.com/RedisTimeSeries/RedisTimeSeries/issues/889