Support FillMode per query

[andresmgot]

Some of the existing data sources (e.g. PostgreSQL) support to specify a fill mode but just for the current query. The $__timeGroup macro of PostgreSQL states: $__timeGroup(dateColumn,'5m', 0), $__timeGroup(dateColumn,'5m', previous), $__timeGroup(dateColumn,'5m', NULL), being the third argument the fill mode.

As far as I can see, using sqlds is only possible to setup the fill mode globally for the datasource.

My suggestion would be to add an optional FillMissing struct to the Query and conditionally use that one (taking precedence over the Datasource fill mode). WDYT @kminehart ?

[kminehart]

Yeah that sounds great! An optional argument makes total sense.