Add HtmlContentPlaceholder for seeding fixture HTML content #1564

Job	Run time
PHPUnit (Symfony ^7.4) (PHP 8.5) (lowest dependencies)	34s
PHP-CS-Fixer (PHP 8.5)	18s
PHPUnit (Symfony 8.1) (PHP 8.5)	47s
Behat (Symfony ^7.4) (PHP 8.5) (lowest dependencies)	4m 11s
Behat (Symfony NEXT 8.2) (PHP 8.5)	4m 24s
PHPUnit (Symfony NEXT 8.2) (PHP 8.5)	40s
PHPUnit (Symfony 7.4) (PHP 8.5)	2m 32s
Behat (Symfony 7.4) (PHP 8.5)	5m 29s
Behat (Symfony 8.1) (PHP 8.5)	4m 55s
	23m 50s

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