Add projection support (and fluent API) to `JdbcAggregateTemplate`

[mp911de]

It would be neat to have projection support, ideally through a fluent API (similar to R2DBC, Cassandra, and MongoDB) to apply projections much closer to the query and conversion instead of fetching and materializing entities and running DTO conversion afterwards.

See also #2098 and #1870 as triggers for this ticket.