Add access to full_path index of uploaded files

[JamminCoder]

Description
Supersedes #7538

Added UploadedFile::getClientPath() method.
This returns the full_path index of uploaded files (when uploaded via directory).
This feature is only available in PHP 8.1+. If the method is called below that version then null is returned.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[JamminCoder]

Ok, let's try again. I fixed all the linting errors, updated the style of my new code, and enabled commit signing, so it should pass the workflow checks.

[JamminCoder]

Hmm, ok. I guess there are still errors. I'll figure those out.

[kenjis]

We still support PHP 7.4. So code must work on PHP 7.4.

[kenjis]

Add a short description in the changelog to let devs know this enhancement.
https://github.com/codeigniter4/CodeIgniter4/blob/4.4/user_guide_src/source/changelogs/v4.4.0.rst#libraries

[JamminCoder]

The 1 failed check is this:

It doesn't look like any of the changes I made would cause it, though I can look into fixing this one too.

[kenjis]

It is because of PHPStan update.
I sent #7542 to develop already.

[JamminCoder]

Ok, great. Is there anything else you need me to do on my end before merging?

[kenjis]

LGTM! Thank you.