[Canvas] Filter is not using the type property correctly; fix in 8.0

[clintandrewhall]

In Canvas, all types used and returned by the Interpreter include a type property, a string, which indicates what the JSON shape represents, (e.g. Datatable has type: 'datatable', etc).

As we were moving the Canvas Function Typescript to the Interpreter package (#36960), we noticed that Filter does not honor this contract. Instead, Filter uses type to indicate what type of filter it is, and is missing the type string.

The fix for this would be to rename the existing type property to something like filterType, and add type: 'filter' to the shape, (and TS type). This would constitute a breaking change for Canvas plugin authors.

We're tracking the need for this change in this issue, as something to ship in v8.0.

cc: @w33ble, @lukeelmers

[elasticmachine]

Pinging @elastic/kibana-canvas

[lukeelmers]

cc @elastic/kibana-app-arch