Fix bug with DB-Forge composite foreign keys

[monkenWu]

Description
Fixes #4310
Original PR #4931

Change the branch base to 4.2.

I edited the addForeignKey method and _processForeignKeys method of CodeIgniter\Database\Forge so that they can define composite foreign keys in an array.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

This looks good.

I was wondering if it would be better to leave the string casting in addForeignKey() method and parse the variables, but using arrays is more natural in this case. I don't think many developers have extended this method so chances that this will break someone's code are low.

I'm fine with these changes, but since we're removing casting in the public method for some variables, please mention this in the changelog (https://github.com/codeigniter4/CodeIgniter4/tree/4.2/user_guide_src/source/changelogs - you will have to create a new file v4.2.0.rst). We don't wanna surprise anyone.

[monkenWu]

@michalsn Thanks for the review.
In fact, I refer to the method addKey(), which accepts both string and array. Therefore, I also continued this pattern in addForeignKey().

I will create a new v4.2.0.rst file soon and mention this change in Enhancements.

[paulbalandan]

Please rename your last commit to something else. We're not releasing yet.

[monkenWu]

@paulbalandan I fixed the composition foreign keys exception and added a new test case

[lonnieezell]

@michalsn are you able to review these changes?

[michalsn]

Thanks @monkenWu it looks good!

One last thing - you have to rebase to resolve conflicts. If you have never done this before, here is a nice instruction: https://samsonasik.wordpress.com/2015/09/16/practical-git-4-rebasing-conflicted-task-branch-against-primary-branch/

[paulbalandan]

@monkenWu can you do a last minute rebase to fix the merge conflicts before we can merge this?