feat(core): specify a Content-Type for individual values when building a formdata request

[RikdeVos]

Description

Based on this discussion: #3315

Support contentType when building a request with formData. Example:

- post:
    name: Post activity
    url: /climate
    formData:
      activity: '{{ activity }}'
      type: '{{ type }}'
      photo:
        fromFile: './photo.png'
      climate:
        value: '{"temperature": "{{ climate.temperature }}", "unit": "{{ climate.unit }}"}'
        contentType: application/json
    expect:
      - statusCode: 201

TODO

• update docs.

Pre-merge checklist

This is for use by the Artillery team. Please leave this in if you're contributing to Artillery.

• Does this require an update to the docs?
• Does this require a changelog entry?

[CLAassistant]

All committers have signed the CLA.

[hassy]

thank you @RikdeVos! we'll review shortly

[hassy]

(failing tests are as expected & due to a secret not being passed in to the job because of security settings in GHA)

[RikdeVos]

Thanks for the quick review @hassy ! Much appreciated