Support custom actions in Herald

Summary:
This was significantly easier than expected.  Here's an example of what an extension class might look like:

```
<?php

final class AddRiskReviewHeraldCustomAction extends HeraldCustomAction {

  public function appliesToAdapter(HeraldAdapter $adapter) {
    return $adapter instanceof HeraldDifferentialRevisionAdapter;
  }

  public function appliesToRuleType($rule_type) {
    return $rule_type == HeraldRuleTypeConfig::RULE_TYPE_GLOBAL ||
      $rule_type == HeraldRuleTypeConfig::RULE_TYPE_OBJECT;
  }

  public function getActionKey() {
    return 'custom:add-risk';
  }

  public function getActionName() {
    return 'Add risk rating (JSON)';
  }

  public function getActionType() {
    return HeraldAdapter::VALUE_TEXT;
  }

  public function applyEffect(
    HeraldAdapter $adapter,
    $object,
    HeraldEffect $effect) {

    $key = "phragile:risk-rating";

    // Read existing value.
    $field_list = PhabricatorCustomField::getObjectFields(
      $object,
      PhabricatorCustomField::ROLE_VIEW);
    $field_list->readFieldsFromStorage($object);
    $field_list = mpull($field_list->getFields(), null, 'getFieldKey');
    $field = $field_list[$key];
    $field->setObject($object);
    $field->setViewer(PhabricatorUser::getOmnipotentUser());

    $risk = $field->getValue();
    $old_risk = $risk; // PHP copies arrays by default!

    // Add new value to array.
    $herald_args = phutil_json_decode($effect->getTarget());
    $risk[$herald_args['key']] = array(
      'value' => $herald_args['value'],
      'reason' => $herald_args['reason']);
    $risk_key = $herald_args['key'];

    // Set new value.
    $adapter->queueTransaction(
      id(new DifferentialTransaction())
        ->setTransactionType(PhabricatorTransactions::TYPE_CUSTOMFIELD)
        ->setMetadataValue('customfield:key', $key)
        ->setOldValue($old_risk)
        ->setNewValue($risk));

    return new HeraldApplyTranscript(
      $effect,
      true,
      pht(
        'Modifying automatic risk ratings (key: %s)!',
        $risk_key));
  }

}
```

Test Plan: Created a custom action for differential revisions, set up a Herald rule to match and trigger the custom action, did 'arc diff' and saw the action trigger in the transcripts.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: locutus, edutibau, ite-klass, epriestley, Korvin

Maniphest Tasks: T4884

Differential Revision: https://secure.phabricator.com/D8784