New PHP Slim4 Server Generator

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

There are breaking changes in Slim 4.0.0 version. In issue #2851 (closed) we agreed to add new generator.

I had to update my own Slim Token Authentication fork because original repo author didn't accept my PRs and seems missing.

Upgrade Guide - Slim Framework

Already contains changes from #3621 Closes #2851 (closed)

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

Questions to maintainers

# START SCRIPT: bin/php-slim4-server-petstore.sh
[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: php-slim4 (server)
[main] INFO  o.o.codegen.DefaultGenerator - Generator 'php-slim4' is considered stable.

1. Generator 'php-slim4' is considered stable seems interesting, where can I change it? This option/flag is not documented as far as I know.
2. What happened with security samples? I remember that I had to run bin/security/php-slim-server-petstore.sh before.
3. Why Openapi 3.0 samples mapped to the same folder where 2.0 samples already exists. I mean ./bin/openapi3/php-slim-server-petstore.sh and ./bin/php-slim-server-petstore.sh scripts targets the same ./samples/server/petstore/php-slim directory.