[Slim4] Data mocker for scalar types

[ybelenko]

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
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I'm working on mock server for Slim generator. Maybe my OpenApiDataMockerInterface can be useful in other PHP generators. Right now only scalar data types implemented. array and object are coming next. Then I'll add enum option to mockString and method mockSchema to generate data from spec schema object.(will be implemented in next PRs)

Related to #3545

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

[ybelenko]

Right now only scalar data types implemented. array and object are coming next. Then I'll add enum option to mockString and method mockSchema to generate data from spec schema object.

I've reconsidered. Small PR will be easier to review. Let's keep this PR with scalar types only, I think it's a good starting point. I'll work on other types in local branch and submit when this will be merged.

[ybelenko]

How to test

1. Checkout into feature branch

$ git checkout -b ybelenko-slim_data_mocker master
$ git pull https://github.com/ybelenko/openapi-generator.git slim_data_mocker

2. Generate new Slim4 samples and go to just created build

$ mvn clean package
$ bin/php-slim4-server-petstore.sh
$ cd samples/server/petstore/php-slim4

3. Start Composer installation and run unit tests

$ composer install
$ composer test
$ composer phpcs

[wing328]

Looks good to me but I've not tested it locally.