Aggregation queries

[weeco]

As a user I'd sometimes like to do aggregations such as:

• Return the number of records that pass my Query / Filter
• Count number of records by a certain condition (e.g. get the number of records where value.firstName == "Martin", number of records per day)
• Sum all value.checkoutPrice from all Kafka records that pass my query/filter

[twmb]

tldr, SQL-like UI in the frontend, SQL-like execution in the backend

[twmb]

If/when we get to this, we can also consider adding filtering by sql, see #517.