Transaction support

[adam-fowler]

Does it make sense to add support for transactions in similar manner to this

extension PostgresClient {
    func withTransaction<Value>(logger: Logger, _ process: (PostgresConnection) async throws -> Value) async throws -> Value {
        try await withConnection { connection in
            try await connection.query("BEGIN;", logger: logger)
            do {
                let value = try await process(connection)
                try await connection.query("COMMIT;", logger: logger)
                return value
            } catch {
                try await connection.query("ROLLBACK;", logger: logger)
                throw error
            }
        }
    }
}