Add AppCheck support

[nhaynes]

Description

We added AppCheck to firebase-php library a little while back and we apparently updated the docs to reflect that it works with laravel-firebase but we never actually made the changes in this project.

Docs referencing the inclusion in laravel-firebase:

Fixes # 171

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have made corresponding changes to the documentation

[codecov[bot]]

Codecov Report

Merging #174 (162b3a5) into main (48b788c) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #174      +/-   ##
============================================
+ Coverage     98.30%   98.38%   +0.08%     
- Complexity       44       46       +2     
============================================
  Files             3        3              
  Lines           118      124       +6     
============================================
+ Hits            116      122       +6     
  Misses            2        2              

Impacted Files	Coverage Δ
src/FirebaseProject.php	100.00% <100.00%> (ø)
src/ServiceProvider.php	100.00% <100.00%> (ø)

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[jeromegamez]

Thank you, it's much appreciated! 🌺

You certainly have noticed that I squashed/rebased your MRs and Commits with different titles. I don't use conventional commits in my projects, and my commit messages are always in the present tense. You don't have to change this, I just wanted to explain why I do it 😅.

[nhaynes]

@jeromegamez I never used to use them either but then I found https://github.com/semantic-release/semantic-release and it convinced me to change my ways. I recommend giving it a go on a future project.