Add migration to/from WAL-based invalidation #8738
                
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To support migration from trigger-based invalidation collection to WAL-based invalidation collection, or the opposite, the function
_timescaledb_functions.set_invalidation_methodis introduced. When called with a hypertable (that has continuous aggregates connected) and an invalidation method, it will flush the hypertable invalidations and changes the collection method for the hypertable to use the requested one.It will changes the collection method for all continuous aggregates connected to the given hypertable, not just for a single continuous aggregate.
Also adds tests for the migration to check that it works properly.