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/dialect/postgres/config.go

@@ -81,9 +81,7 @@ func DecodeConfig(input any) (database.Connector, error) {
 		if err = decoder.Decode(c); err != nil {
 			return nil, err
 		}
-		return &Config{
-			Config: &pgxpool.Config{},
-		}, nil
+		return connector, nil
 	}
 	return nil, errors.New("invalid configuration")
 }

backend/v3/storage/database/dialect/postgres/pool.go

@@ -2,8 +2,10 @@ package postgres
 
 import (
 	"context"
+	"database/sql"
 
 	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/jackc/pgx/v5/stdlib"
 
 	"github.com/zitadel/zitadel/backend/v3/storage/database"
 	"github.com/zitadel/zitadel/backend/v3/storage/database/dialect/postgres/migration"
@@ -13,7 +15,13 @@ type pgxPool struct {
 	*pgxpool.Pool
 }
 
+// RawDB implements [database.PoolTest].
+func (p *pgxPool) RawDB() *sql.DB {
+	return stdlib.OpenDBFromPool(p.Pool)
+}
+
 var _ database.Pool = (*pgxPool)(nil)
+var _ database.PoolTest = (*pgxPool)(nil)
 
 func PGxPool(pool *pgxpool.Pool) *pgxPool {
 	return &pgxPool{