Support form-data arrays

[hardillb]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Add the ability to add multiple entries with the same name using an array to a mulitpart form data post as described in this Stack Overflow post:

https://stackoverflow.com/questions/74904546/node-red-send-post-request-and-using-form-data

Currently the array is stringified and added as a single entry in the body.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage: 68.263% (+0.006%) from 68.257% when pulling 4c1d7ad on hardillb:http-request-form-array into 113d42e on node-red:master.

[knolleary]

It has been noted in #4279 that this causes a change in behaviour that is going to break existing flows.

Give this PR was an attempt to add support for a scenario we don't currently support, I think we can aim to find a way to support it in a more backwards compatible way. As such, I'm going to revert this PR.