feat: #2870 SOAP request with attachments

[vyletpet]

Description

Added possibility to set content type as "multipart/related" to be able to support SOAP with attachments (SwA} requests.

Motivation and Context

To be able to send SwA requests. https://github.com/apache/jmeter/issues/2870

How Has This Been Tested?

Extended unit tests

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.