Add support for core dynamic_form

[sumaiyamannan]

Hi,

This fixes the issue 104 and 112

To replicate the issue do the below:
Add multiblock plugin to a Moodle 4.2 site and add a block
Go to 'Manage contents' and click on the wheel icon to edit the block instance

It will lead to

Fatal error: Cannot override final method core_form\dynamic_form::__construct() in blocks/multiblock/classes/form/editblock_base.php on line 60

Apply the change and see that the issue is resolved

Note: This is for Moodle site 4.2 and above only.

Cheers,
Sumaiya