Allow comments to use expressions to select target rows.

[okennedy]

current behavior: Comments can target individual rows.

desired behavior: When the row field of a comment cell begins with an =, treat the remaining characters as a SparkSQL expression identifying a set of rows to target.

This functionality is already supported in the Mimir-API (via the 'condition' field of the lens configuration), and we would just need to add logic to the Vizier-API to pass the appropriate field at the right time.