Feature: #87200 - Send plaintext and HTML mails in EmailFinisher

See Issue #87200

Description

The EmailFinisher of EXT:form now sends mails with a plaintext and an HTML part. A new option addHtmlPart has been added to configure if a HTML part should be added to mails.

Impact

Mails now contain both plaintext and HTML parts to support a wider audience.

Enforcing a plaintext-only mail increases reception and security.