Refactor view management

[simolus3]

This replaces intermediate SQL logic used to setup views with a simple filter implemented in Rust. This makes the flow much easier to understand, and avoids re-parsing tables from JSON multiple times.

Specifically, this removes the following internal SQL functions and views:

• The powersync_views view and associated triggers.
• powersync_exec
• powersync_view_sql, powersync_trigger_delete_sql, powersync_trigger_insert_sql, powersync_trigger_update_sql.

Instead, it adds the ExistingView struct which holds the same fields as powersync_views and uses the same query to read views. When we apply a schema, we use the parsed schema struct to find necessary updates instead of expressing that in SQL. The functions to create SQL for views and triggers has also been refactored to be Rust-internal.

I've kept powersync_drop_view because it's used in a down migration we can't change. I had to migrate one up migration to use the new direct Rust calls in favor of the SQL layer, but the behavior should be the same.

These changes are covered by existing tests. This still keeps powersync_tables around, I will refactor that away in a follow-up PR.

[rkistner]

Nice, this does appear to simplify the management