Reference introduction with EntryReference and StrucureReference implementations

[norberttech]

Change Log

Added

• Reference - representing reference to given entry/entries in DataFrame

Fixed

Changed

Removed

Deprecated

Security

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Description

The goal of Reference is to stop using primitive string values as entry references and move into Reference object that might come with things like alias, for example.

The final goal is to replace all string $entry with string|Reference $entry types.
string should still stay for convenience in simple use cases.

Example:

<?php

declare(strict_types=1);

use function Flow\ETL\DSL\struct;
use Flow\ETL\DSL\CSV;
use Flow\ETL\DSL\Transform;
use Flow\ETL\Filesystem\SaveMode;
use Flow\ETL\Flow;
use Flow\ETL\GroupBy\Aggregation;

require __DIR__ . '/../../bootstrap.php';

(new Flow())
    ->extract(CSV::from(__FLOW_DATA__ . '/users.csv'))
    ->rows(Transform::array_unpack('row'))
    ->groupBy('user_id')
    ->aggregate(
        Aggregation::collect(struct('skill', 'value', 'active')->as('skills')),
    )
    ->rows(Transform::to_json('skills'))
    ->write(CSV::to(__FLOW_OUTPUT__ . '/users_aggregated.csv'))
    ->mode(SaveMode::Overwrite)
    ->run();