Support DUPLICATE_POLICY on ts.add

[dpipemazo]

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: RedisTimeSeries/RedisTimeSeries#889