Require ```--retention``` property for ```ts.add``` or ```ts.create``` Redis command when importing

Hi ,here are the available commands in the latest release below,but I can't find retention field,so may there be any alternative RIOT solution(not lua or others)?Since we have timeseries-level properties such as on-duplicate and label,we also have expire commands.

Add samples to RedisTimeSeries
  -h, --help                Show this help message and exit.
      --ignore-missing      Ignore missing fields.
  -k, --key=<fields>...     Key fields.
      --keyspace=<str>      Keyspace prefix.
      --labels=SPEL...      Labels in the form label1=field1 label2=field2...
      --on-duplicate=<name> Duplicate policy: BLOCK, FIRST, LAST, MIN, MAX, SUM (default: LAST).
      --remove              Remove key or member fields the first time they are used.
      --separator=<str>     Key separator (default: :).
      --timestamp=<field>   Name of the field to use for timestamps. If unset, uses auto-timestamping.
  -V, --version             Print version information and exit.
      --value=<field>       Name of the field to use for values.

redis / riot

Require ```--retention``` property for ```ts.add``` or ```ts.create``` Redis command when importing #151