R1:711871f6bc74

Allow typeaheads to pass nonscalar data to datasources

Summary:
Ref T13250. Currently, datasources have a `setParameters(...)` method. This method accepts a dictionary and adds the key/value pairs to the raw HTTP request to the datasource endpoint.

Since D20049, this no longer works. Since D20116, it fatals explicitly.

In general, the datasource endpoint accepts other values (like `query`, `offset`, and `limit`), and even before these changes, using secret reserved keys in `setParameters(...)` would…