Create Trigger / Drop Trigger on distributed tables

We currently don't have native support built-in for distributed tables. Users who want to use triggers do so by manually creating them on a distributed table's shards on the worker nodes.

I'm opening this issue to track adding `CREATE TRIGGER` and `DROP TRIGGER` support on distributed tables. I'm also listing a few use-cases that relate to trigger support below.
1. Propagate `CREATE TRIGGER` to all shards and shard placements: This could for example be useful in the context of full text search.
2. Propagate `CREATE TRIGGER` to all shards but only one of the shard placements: #228 and the mailing list update there talks about an example usage.
3. Propagate `CREATE TRIGGER` to pass along a notification: For example, an IoT database may need to alarm if the underlying sensor reports that the battery's charge rate < 5%.
4. Ingest from source table to colocated destination table: In this example, the source table stores raw events, and the destination table has aggregated stats. The user creates a trigger to roll up raw events from the source table, and put them into the destination table.