This aims to improve cooperation between `flexmailer`, `civicrm-wordpress:wp-rest/`, and
civicrm/civicrm-core#17312 -- so that flexmailer produces compliant URLs.

Before
------

The open-tracking URL is constructed as a reference to the standalone script `extern/open.php`
using certain `$config` properties.

After
-----

On Civi v5.23+, it uses `CRM_Utils_System::externUrl()` API to request the full URL of
`extern/open`.  Flexmailer is no longer responsible for figuring the URL, and other
agents (via `hook_civicrm_alterExternUrl`) can do so.

On Civi v5.22 and earlier, it continues using the same old formula. This provides
drop-in/bug-level-compatibility on older versions. This can be removed in a couple months.

Comments
--------

I suspect this also fixes [dev/mail#17](https://lab.civicrm.org/dev/mail/issues/17),
wherein the open tracker has flawed URLs in certain multilingual configurations.
However, I can't confirm for certain because I don't have that configuration.

But just to game this out...

* If the current patch fixes dev/mail#17, then huzza!
* If it doesn't, then the problem is in `civicrm-core`'s `CRM_Utils_System::externUrl()`
  (not `flexmailer:OpenTracker.php`). Which means:
    * The problem already affects other use-cases in `civirm-core` in v5.23+.
    * The fix should go in `civicrm-core`.

Either way, addressing dev/mail#17 shouldn't require any further change in `flexmailer`.

Generally, I think the `is_callable()` formulation is better.  However, this
specific class (`CRM_Utils_System`) defines a `__callStatic()` magic-methods,
which prevents `is_callable()` from being meaningful.

So here's Plan B.