php-laravel, enum models, nullable and default values

[wing328]

• support for enum models & separating mustache templates for enums and generics
• extract property 'type' and 'default value' from a #ref to an enum model
• support for PascalCase naming convention for PHP generators
• use a default value for known basic types when they are not nullable
• use 'null' as default value for nullable types when they do not specify the default value
• use defined constant path as enum default value when found

credits: @aghajani

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 (6.0.1) (patch release), 6.1.x (breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)

[wing328]

Tested locally:

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[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (bundle-copy-config) @ LaravelPetstoreServerTests ---
[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (bundle-application-key-set) @ LaravelPetstoreServerTests ---
Application key set successfully.
[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (bundle-test) @ LaravelPetstoreServerTests ---
PHPUnit 8.5.26 #StandWithUkraine

..                                                                  2 / 2 (100%)

Time: 206 ms, Memory: 18.00 MB

OK (2 tests, 2 assertions)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  29.349 s
[INFO] Finished at: 2022-05-27T11:06:04+08:00
[INFO] ------------------------------------------------------------------------