feat: test event reduction to relational tables (#11718)

# Which Problems Are Solved

This PR provides an example for testing DB reduction to relational
tables without exploiting integration tests.

# How the Problems Are Solved

- Set up a disposable embedded Postgres DB used to store the results of
the calls to the reducers and verify such results
- Add utility method to return transactions from the existing DB pool.
These transactions can be used by the single test cases to insert and
read data from DB
- Add a test to verify that the handler contains the expected reducers
(checking also that they are on the right aggregate type)
- Add tests to verify the single reduce functions. A parent test creates
the required objects to satisfy the foreign key constraints.

# Additional Changes

  - Typo fixed in `postgres.DecodeConfig()`
  - Discard unneeded `projectionName` parameter for `Executor` function
 
# Additional Context

- Closes #11430

backend/v3/storage/database/database.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"context"
+	"database/sql"
 )
 
 // Pool is a connection pool. e.g. pgxpool
@@ -20,6 +21,9 @@ type PoolTest interface {
 	Pool
 	// MigrateTest is the same as [Migrator] but executes the migrations multiple times instead of only once.
 	MigrateTest(ctx context.Context) error
+	// RawDB returns an *[sql.DB] handle backed by the same database as the [PoolTest].
+	// The returned handle is intended for use in tests, and the caller is responsible for closing it when done.
+	RawDB() *sql.DB
 }
 
 // Connection is a single database connection which can be released back to the pool.