feat: add embedded testing server for postgres (#9955)

# Which Problems Are Solved

1. there was no embedded database to run tests against
2. there were no tests for postgres/migrate
3. there was no test setup for repository which starts a client for the
embedded database

# How the Problems Are Solved

1. postgres/embedded package was added
2. tests were added
3. TestMain was added incl. an example test

# Additional Changes

none

# Additional Context

closes #9934

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/v3/storage/database/migration.go

@@ -4,5 +4,6 @@ import "context"
 
 type Migrator interface {
 	// Migrate executes migrations to setup the database.
+	// The method can be called once per running Zitadel.
 	Migrate(ctx context.Context) error
 }