Headers should allow "multiple" entries

Problem/Motivation

Users should be able to provide multiple Header entries, however users are currently only allowed to specify one.

Steps to reproduce

Attempt to add two headers for an HTTP request, the UI is currently a textfield making it impossible to do so.

Proposed resolution

Switch the header context definition to support "multiple", however a migration pathway for updating existing Rules via a post_update hook → will be necessary to ensure that sites with existing single header fields are converted into an array.

It'll result in a fatal exception otherwise.

Remaining tasks

Provide issue fork/patch.

User interface changes

Header text field will be converted to a textarea, and allow users to specify multiple.

Data model changes

The Header data field will be converted from a string to an array of strings.