Add a OrderCurrency condition

The attached patch is missing a schema and a test.

Other comments:

+    $active_currencies = $this->currencyStorage
+      ->loadByProperties([
+        'status' => TRUE,
+      ]);

This line is not formatted right, we don't break method calls to the next line like this.
Also, there is no need to load by properties, currencies don't use a status.

+    // If no currency is imported or active, return no form.
+    if (empty($active_currencies)) {
+      return $form;
+    }

We generally don't do this. Without a currency you can't create a store, and without a store you can't create most other entities, so we're protected.

+    foreach ($active_currencies as $currency_code => $currency) {
+      /* @var \Drupal\commerce_price\Entity\Currency $currency */
+      $currency_list[$currency_code] = $currency->getName();
+    }

You can use EntityHelper::extractLabels($currencies) instead.

+    $selected_currencies = $this->configuration['currencies'];

No point in introducing this variable when it's only used once.

+    $form['currencies'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Currencies'),
+      '#multiple' => TRUE,
+      '#default_value' => $selected_currencies,
+      '#options' => $currency_list,
+      '#required' => TRUE,
+    ];

Let's use a commerce_entity_select element here, so that it uses checkboxes for up to 7 elements, and an autocomplete for more than that.
Less painful than a multiple select widget. Plus, it matches what we do in the OrderStore condition.

+    $this->configuration['currencies'] = $values['currencies'];

Without an array_filter() the currencies key will also contain unselected currencies.

+    return in_array($order_currency, $selected_currencies) ? TRUE : FALSE;

in_array already returns a boolean, the ternary is completely unneeded.

Added test, schema, addressed my own feedback, moved the condition to commerce_order.

Thanks for getting the ball rolling, Sumanth!