Enhancement: Add the info requiring PHP-XML extension for Jetpack connection

#### Is your feature request related to a problem? Please describe.

Sometimes, when we run cURL requests to xmlrpc.php files, we see this error:

```
[0] => -32700
[1] => parse error. not well formed
```
Most of the times, this error is an indicator that the site is missing [the php-xml extension](http://php.net/manual/en/xml.installation.php).

#### Describe the solution you'd like

I think we can check if a site does not have this extension, then displays a warning in WP Admin.

#### Additional context

* Mentioned in the internal document PCYsg-hOY-p2. Fixed 2 issues there.
* Fixed in 1648354-zen
* https://wordpress.org/support/topic/error-code-32700-2/#post-10592484

The issue does not name a file or test. Start by locating the Jetpack connection handling for XML-RPC responses and existing WP Admin extension warnings. Done means detecting when PHP-XML is unavailable and displaying a warning in WP Admin for the site.

Written by the indexing model from the issue text.