[php-slim4] Add minimal GitHub Action config

[ybelenko]

I used that example as reference.
The most interesting part is that I've checked it with a fresh repo, output:
First run without coding style check
Second run, coding style check on

I think that failed CI(links above) is absolutely normal regarding this PR, since coding style fix should be added in next PRs. The purpose of this change is to generate CI config which launches GitHub runners without errors(correct syntax).
Relative issue #11858

cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @renepardon

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

@ybelenko thanks for the PR.

FYI. I did something similar for the Java client with #11633 before but we didn't check the code style.