Add custom mailer command for Drupal Symfony Mailer

[elgandoz]

Description

The Drupal Symfony Mailer module has become the de-facto solution for managing HTML emails in Drupal 10, and there are ongoing discussions about incorporating Symfony Mailer into Drupal core (Issue #3380476). To align with this shift, I suggest adding the necessary configuration adjustments to the Drupal 10 Platform.sh template.

This PR adds a custom command, that will need to be selected in the configuration when creating a Sendmail transport as described in the official documentation.
NB: the parameters -t -i are not necessary since they're inherited by the sendmail_path.

Related Issue

#85

Motivation and Context

It provides a piece of configuration required and that is Platform.sh specific, for a module that is not in core but likely installed.
https://www.drupal.org/docs/contributed-modules/drupal-symfony-mailer/getting-started#s-platformsh

How Has This Been Tested?

HTML emails are being sent on my Platform.sh instance.

Screenshots (if appropriate):

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

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• I have read the contribution guide
• I have created an issue following the issue guide
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.