Add a new Salesforce Push Upsert Allowed Event

Problem/Motivation

When no Salesforce ID is set, Upsert is required on push, but in some cases this is undesired.

We have a case where a Lead is converted to a Contact and then the Contact subsequently had their email address changed, in which case we can prevent the push with SalesforcePushAllowedEvent but actually we may want the push to still occur to create a new Lead in this case rather than Upsert into a converted lead (which would result in error code "CANNOT_UPDATE_CONVERTED_LEAD" from Salesforce). Yet the below code does not allow us to skip the upsert and instead go the 'create' route.

MappedObject::push contains:

    if ($this->sfid() && !$mapping->alwaysUpsert()) {
      $action = 'update';
      $result = $this->client()->objectUpdate(
        $mapping->getSalesforceObjectType(),
        $this->sfid(),
        $params->getParams()
      );
    }
    elseif ($mapping->hasKey()) {
      $action = 'upsert';
      $result = $this->client()->objectUpsert(
        $mapping->getSalesforceObjectType(),
        $mapping->getKeyField(),
        $mapping->getKeyValue($drupal_entity),
        $params->getParams()
      );
    }
    else {
      $action = 'create';
      $result = $this->client()->objectCreate(
        $mapping->getSalesforceObjectType(),
        $params->getParams()
      );
    }

Steps to reproduce

1. Have a mapping for Lead <> Drupal User with upsert key as 'Email'
2. Have a mapping for Contact <> Drupal User
3. Register a new user with e.g. me@example.com
4. Convert a Lead in SF, change its email address to me2@example.com
5. Sync down Contact <> Drupal User (assuming Lead to Contact conversion is handled in your code)
6. Register a new user with me@example.com, ie, the same email address of the previously converted Lead
7. New Mapping is created for (6) yet the Upsert behaviour will kick in on upsert key. But the Lead with that email is already converted. We instead want a new Lead.

Proposed resolution

Add a new event

Remaining tasks

MR

User interface changes

N/A

API changes

New SalesforcePushUpsertAllowedEvent

Data model changes

N/A