[Proposal] Add warning if no php-soap installed

[dmitrydonskih]

This extension requires package php-soap to run.
Joomla by default does not require this package.
So in an environment where php-soap package is not installed, an attempt to test a connection leads to fatal error: Class 'SoapClient' not found.

Tested on Joomla! 4.2.2, PHP 8.2.24, LogicalDOC Joomla explorer 4.1.0.

Propoposed solution would be to check existence of class SoapClient during installation of component and display a relevant warning message if none found.