Add workflow data subject & field interfaces

A subject contains multiple fields. Each field has a type and they
can be filtered and used later on.

[MAILPOET-4136]

mailpoet/lib/Automation/Engine/Workflows/Field.php

@@ -0,0 +1,60 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Workflows;
+
+class Field {
+  public const TYPE_BOOLEAN = 'boolean';
+  public const TYPE_INTEGER = 'integer';
+  public const TYPE_STRING = 'string';
+  public const TYPE_ENUM = 'enum';
+
+  /** @var string */
+  private $key;
+
+  /** @var string */
+  private $type;
+
+  /** @var string */
+  private $name;
+
+  /** @var callable */
+  private $factory;
+
+  /** @var array */
+  private $args;
+
+  public function __construct(
+    string $key,
+    string $type,
+    string $name,
+    callable $factory,
+    array $args = []
+  ) {
+
+    $this->key = $key;
+    $this->type = $type;
+    $this->name = $name;
+    $this->factory = $factory;
+    $this->args = $args;
+  }
+
+  public function getKey(): string {
+    return $this->key;
+  }
+
+  public function getType(): string {
+    return $this->type;
+  }
+
+  public function getName(): string {
+    return $this->name;
+  }
+
+  public function getFactory(): callable {
+    return $this->factory;
+  }
+
+  public function getArgs(): array {
+    return $this->args;
+  }
+}