Adds strict types and warning in ci

[jcchavezs]

Short description of what this PR does:

• Enables warnings on CI
• Adds declare(strict_types=1); in all PHP files.

Checklist

• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the [Contribution Guide] and my PR follows them.
• [x] I updated my branch with the master branch.
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation about the functionality in the appropriate .md file
• [ ] I have added in line documentation to the code I modified

Ping @cawolf @piotrooo @mszabo-wikia

[jcchavezs]

I don't seem to get the warnings here. Any clue @mszabo-wikia @piotrooo ?

[mszabo-wikia]

@jcchavezs I think this is because MockTracerTest::testInjectSuccess uses an injector callable that expects $carrier to be passed by value, rather than by reference, so it causes no warning. The warning is triggered when we change it to expect $carrier by reference (like a real-life injector would do).