feat(query): arbitrary target-schema columns

[alextheimer]

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Currently, target-schema "filter" labels (i.e. those used to identify whether-or-not a target-schema is configured against a series) can include only shard-key columns. This PR adds support for arbitrary target-schema filter labels.

For example, a dataset with shard-key columns SA and SB can now have a target-schema defined against an arbitrary column C:

{SA=foo, C=bar}      // filter labels
-> [label1, label2]  // sharding labels

See here for an example spec.